Look out for vandalism, and revert it. It is often worthwhile to check the page history after reverting to make sure you have removed all the vandalism. Also, check the user contributions of the vandal - you will often find more malicious edits.
Additionally, leave warning messages on the vandal's talk pages using the following system.
Note that these templates need not be used sequentially. If the edit is clearly vandalism, use the template {{uw-vandalism1}} instead. For continuing severe vandalism, {{uw-vandalism2}} may be skipped and {{uw-vandalism3}} given straight after the first warning. If, however, you are not sure that the edit is vandalism, always start with {{uw-test1}}. If a user made such an edit and reverted it himself, use a {{selftest}}. For extreme or extensive cases of vandalism committed by users who have received no prior warnings, {{uw-vandalism4im}} may be used. The ~~~~ in the templates below cause the time and your signature to be added to the warning.
{{subst:selftest}} ~~~~ (use if an edit appears to be a reversion of a newbie test)
Welcome to Wikipedia. Thank you for reverting your recent experiment. Please take a look at the welcome page to learn more about contributing to our encyclopedia. If you would like to experiment further, please use the sandbox instead. Thank you.
{{subst:uw-test1}} ~~~~ (use if an edit appears to have possibly not been an intentional act of vandalism)
Welcome and thank you for experimenting with Wikipedia. Your test worked, and it has been reverted or removed. Please take a look at the welcome page to learn more about contributing to this encyclopedia. If you would like to experiment further, please use the sandbox instead. Thank you.
{{subst:uw-vandalism1}} ~~~~ (use if an edit is clearly vandalism)
Welcome to Wikipedia. Although everyone is welcome to contribute to Wikipedia, at least one of your recent edits did not appear to be constructive and has been reverted or removed. Please use the sandbox for any test edits you would like to make, and read the welcome page to learn more about contributing constructively to this encyclopedia. Thank you.
{{subst:uw-unsor1}} ~~~~ (use if an edit is unreferenced)
Welcome to Wikipedia. Everyone is welcome to contribute to the encyclopedia, but when you add or change content please cite a reliable source for the content of your edit. This helps maintain our policy of verifiability. Take a look at Wikipedia:Citing sources for information about how to cite sources and the welcome page to learn more about contributing to this encyclopedia. Thank you.
Please refrain from making test edits in Wikipedia pages, even if you intend to fix them later. Such edits appear to be vandalism and have been reverted. If you would like to experiment again, please use the sandbox. Thank you.
{{subst:uw-delete2}} ~~~~ (a variant suitable for vandalism that consists of blanking text)
Please do not remove content or templates from pages on Wikipedia without giving a valid reason for the removal in the edit summary. Your content removal does not appear constructive, and has been reverted. Please make use of the sandbox if you'd like to experiment with test edits. Thank you.
Please stop making test edits to Wikipedia. It is considered vandalism, which, under Wikipedia policy, can lead to a loss of editing privileges. If you would like to experiment again, please use the sandbox.
{{subst:uw-vandalism4}} ~~~~ (not suitable as a first warning)
This is the final warning you will receive regarding your disruptive edits. If you vandalize Wikipedia again, you may be blocked from editing without further notice.
{{subst:uw-vandalism4im}} ~~~~ (used as a first warning for extreme cases of vandalism)
This is the only warning you will receive regarding your disruptive edits. If you vandalize Wikipedia again, you may be blocked from editing without further notice.
Alternatively, you can use:
{{selftest-n|PageName}} ~~~~
{{subst:uw-test#|PageName}} ~~~~
{{subst:uw-vandalism#|PageName}} ~~~~
{{subst:uw-delete#|PageName}} ~~~~
{{subst:uw-vandalism4im|PageName}} ~~~~
{{subst:uw-unsor#|PageName}} ~~~~
to explicitly state which articles were vandalized. Where appropriate, replace the "#" with the warning level number (1 to 4). Insert the title of the article being referenced. For example:
{{subst:uw-test1|France}} ~~~~
Welcome, and thank you for experimenting with Wikipedia. Your test on the page France worked, and it has been reverted or removed. Please take a look at the welcome page to learn more about contributing to this encyclopedia. If you would like to experiment further, please use the sandbox instead. Thank you. Gsmgm (talk) 14:45, 18 February 2009 (UTC)
The "subst" causes the template text to be pasted into the talk page as if you had typed it out, instead of leaving {{subst:uw-test1}} visible when editing the page. This makes the messages more personal to the user, and thus, more friendly. Also, if someone vandalizes the template, then the vandalism will not affect every page that uses the text from the template.
If the vandal will not stop, list them at Wikipedia:Administrator intervention against vandalism. Ensure that the user has been warned thoroughly before posting a notice on Wikipedia:Administrator intervention against vandalism and has had time (three or four minutes) to read the warnings and still ignore them. If a user has not been sufficiently warned, or has only vandalised a couple of times an administrator may simply remove the notice without action.
The blocking administrator will usually leave this on the vandal's talk page:
Welcome to Wikipedia. Thank you for reverting your recent experiment. Please take a look at the welcome page to learn more about contributing to our encyclopedia. If you would like to experiment further, please use the sandbox instead. Thank you.
Please stop making test edits to Wikipedia. It is considered vandalism, which, under Wikipedia policy, can lead to a loss of editing privileges. If you would like to experiment again, please use the sandbox.
This is the final warning you will receive regarding your disruptive edits. If you vandalize Wikipedia again, you may be blocked from editing without further notice.
