Sysop vandalism
Sysop vandalism is degrading the quality of a blog, newsgroup or wiki for reasons that have nothing to do with the objective of the project or the mandate of the group. It occurs frequently on Wikipedia and Disinfopedia - it is worst on the latter where sysops tend to delete things with no process at all, and ignore the votes for deletion process that non-sysops are required to go through. This is a very severe long-term problem and plays a major role in many worst cases visible on other public wikis.
Such vandalism is surprisingly common: Wikipedia permits and encourages sysop vandalism with the universal excuse that "trolls" were responsible for the edits, and that somehow they will be discouraged by being "punished" somehow - this idea that punishment works is part of a carceral state metaphor. This is the basic conceptual metaphor of Wikipedia and other wikis that insist one use real names (and thus be subject to offline abuse by sysops and their friends). These problems are made much worse by permanent sysop status and a model where one pays no price and loses no status even for the reversion of edits which are deemed ultimately constructive. Such behavior is certain to drive off the best contributors, but to serve the sysops' purpose of "converting" the board or wiki into a virtual community only for their own friends.
Typically such vandalism is a symptom of conflicts between users in which the sysop is not neutral, that is, they wish to encourage one contributor and discourage another. They abuse their sysop powers by banning "those who their friends do not like", and eliminating valid contributions towards the goals. There may be cases where this is valid, i.e. someone irreplaceable has made clear that they will not participate if someone else is tolerated. Unless the medium has a formal power structure, e.g. it's a political party large public wiki governed by the policies of that party and its officers, such decisions are almost always either "sysop instinct" or GodKing choices.
A specific sysop power structure may be required to reduce such vandalism.
A related issue is sysop vigilantiism which is not necessarily vandalism, but does subvert due process and degrade trust in the power structure.