I noticed that username 'repecmps' is no longer clickable, for example in one of his/her answer here. Is it because he/she has been banned from JLU? Do we even have a user banning mechanism here?
Just for the record: the Stack Exchange platform has no concept of a "banned" account. People like to throw the word around, but to the best of my knowledge, no one has ever been banned and I've never seen anything in the software implying that the ability exists.
Users can be suspended, i.e., told to take some time away from a site. Suspended users get a (publicly viewable) date on which they can resume participating.
Users can, if they choose, request that their accounts be deleted. Why a user would want to do this is their own business, and we'll try to talk people out of it if we think they're a net positive to the community, but they can and do request this. That's what happened in this particular case.
Unclickable username == already deleted account.