JDelekto: I wouldn't go for the "pay for play", only because it tends to assume only the "rich elite" could post; however, I think an ignore button would be quite apropos.
Beware the 'ignore', a person seeking attention who does not get it could become a threat to themselves or others.
Magmarock: What makes an ignore button better then a block button is that it's subtle. A block button lets the person know they are blocked and some people will attempt to get around that. The reason I suggested a pat to post, was so that there was some measure to stop people from joining to just post nonsense.
An ignore may be more 'subtle', but it wouldn't take someone long to catch on if you limit their ability to do anything.
Better yet, (and maybe this was your intent) if you use 'ignore', how about hiding any posts, threads or messages in which the 'ignored' person posted? This way, you're not necessarily shunning them, but you're opting in to not see anything from them.