Sysop vigilantiism is the process equivalent of sysop vandalism. As sysop vandalism degrades the material using the sysop power structure's bias, sysop vigilantiism degrades the trust in that structure itself. It is a major problem at Wikipedia.
The term was actually coined by Wikipedia's GodKing, "Jimbo" Wales.
" I am not aware of any discussion which has reached a rough consensus in favour of allowing sysops to block "obvious trolls". Rather, it appears to me that some sysops have decided amongst themselves to act as "vigilantees"." - Martin Harper
See also the more serious developer vigilantiism.