There is mods for making the site appear to be working badly like really slow page loads for most forum software.
If they are absolutely persistent there is not much you can do. The best suggestion I've come across is to modify the software so any post they make is only viewable to them or moderators.
Your other members will never see them and hopefully the poster doesn't catch on, as long as they aren't getting any attention which is what most of them are seeking they will go bother someone else eventually.
[edited by: thecoalman at 3:12 pm (utc) on Oct 24, 2010]
Hoople, a violation of terms would be straightforward... I was asking here because some cases are borderline. Can't be considered spam, poster does not violate any rule, yet manages to annoy many members with his trolling. But it is masqueraded enough that it falls in a gray area.
This forum script is old and has none of the more advanced features described by all of you. I would actually hate to make someone "invisible" as thecoalman and maximillianos suggested but this gave me another idea. Perhaps just altering the alerts sent after he posts will suffice. Cutting that line will kill a lot of the noise and stop bringing in other members to confront him. So not invisible, but close.