Vandalism is any edit made to a page that is intentionally nonconstructive. Good examples are removing content, replacing it with nonsense, inserting false information, or adding loud, unhelpul messages (e.g., "THE WORLD SUCKS!"). Any attempt to help or add in any way with good intentions but leaves a page messy or unhelpful is not vandalism, but merely work by an inexperienced user who wants to help but needs a push in the right direction. If this is the case, revert their edits and leave them a message explaining what was wrong. If they do not respond or give any sign that they read and understand the message, give them as many warnings as may seem appropriate. If they are still apparently not listening, ban them from the wiki.
What to do
If you are an administrator, revert/delete their edits, then block them immediately and permanently. There's no need to talk to them, for various reasons, including the fact that they...
- Won't listen
- Want you to be annoyed and will be happy about messages telling them to stop
- Are usually automated anyways.
If they have made constructive and helpful edits, tell them to stop doing the bad ones as warning. If they continuously persist, then ban them, either temporairly or permanantly, depending on what sorts of changes the implemented.
If the offender is an anonymous contributor, then do not block them forever. Just block them for maybe 6 months or a year.
If you are just a user and do not have such prvileges as blocking people, then simply revert the edits. If they create entirely new pages for their purposes, then insert the delete template into the page (s). Then let an administrator know.