Ad hominem

Ad hominem is the weakest form of argument. It is an attack on an argument based on who made the argument, rather than based on its merits.

Ad hominem revert is an obvious example, one common on large public wikis and giving rise to calls for a revert currency to make such actions "not free". Ad hominem delete is an example of this weak form raised to sysop vandalism, a particularly stupid phenomenon encouraged mostly by Wikimedia.