Community moderators already have an ability to mark potential users as troublemakers via annotations, but that ability isn't very useful in itself.
It would be nice if mods could mark users as potential troublemakers and then could see a page with a summary of all the latest activity by these marked users. The ability to remove the mark would also be appropriate in this case, if the users changed their ways after receiving a few comments / e-mails / death threats.
A basic use-case: a user's only two answers contain a link to her own website, but they appear in relevant questions. This isn't spam and is fine by me, but I would like to keep that user under the spotlight for a few days to watch if she continues to do this further, and maybe leak into questions which aren't as relevant - this has recently happened on Gaming.
Alternatively, if adding a new specialized page for this isn't considered beneficial, maybe we should have the low-post-quality detector add "bad points" to posts by these users, so that it's more likely low-quality posts from these users would float to the top of the problematic-posts list.