Vandalism is the act of editing any SmashWiki article intentionally for the purpose of reflecting the site in a negative light by putting its integrity into question. Vandalism is taken seriously on SmashWiki: first-time offenders will almost always receive a stern warning from another user, while repeat vandals will be blocked from editing for a period of time as determined by a sysop or bureaucrat.
Vandalism vs. Erroneous editing
Any edit that is considered to have been made to intentionally compromise SmashWiki's integrity is considered an act of vandalism; mistaken edits generally do not fall under this category. In most cases, there is a clear difference between vandalizing an article and inadvertently or unintentionally adding incorrect or inappropriate material. For example, if one were to edit the Mr. Game & Watch (SSBM) article and add a line of text suggesting that Judgment is the most powerful attack in the game (due to the chance of dealing a one-hit KO when performing the move), that would be considered good faith, not vandalism; however, an edit of the article to read "Mr. Gay & Watch is the suckiest character in Melee" quite clearly would be.
Dealing with vandalism
If you encounter a vandalized article on SmashWiki, there are several things that you can do. The most immediate course of action should be to revert any edits to the article that you identify as vandalism. Afterwards, you may want to check the suspected vandal's contribution history to identify any other possible cases of malicious editing. If you determine it to be an isolated case (i.e. the suspected user hasn't made any further or past malicious edits), you might want to send them a message politely reminding them of SmashWiki's vandalism policy. If it appears that the user is, in fact, a dedicated vandal, you should send a message to a sysop informing them of the situation on their talk page or by using the admin noticeboard; a listing of all sysops may be found here.
From there, the sysop can do some investigating of their own to determine if the user warrants blocking. Generally speaking, most suspected vandals usually do end up being blocked for some time, with the more extreme and dedicated among them being blocked indefinitely. If you suspect a SmashWiki user of being a vandal on other NIWA wikis, notify an admin.