I have been invited by one serial inviter (see this post of mine).
I have had a total of three spam comments (now deleted).
Y'know, I feel rather left out here.
The serial inviter in question was x_____________x. The community appeared to be (before it was suspended) full of people moaning about why they had been added. People who had gone ahead and clicked the link in the e-mai they received, without looking to see what would happen. This e-mail clearly states that "If you do not wish to be added to x_____________x, just ignore this email". Never mind that communities are opt-in and you can opt-out at any time. You can't be added to a community that you don't want to be added to, and no-one in that community can stop you leaving.
Yet these people who are capable of using an e-mail client (and indeed LiveJournal) failed to realise that.
Trolls you can't do much about - even the abuse team's not bothered. They're not harassing anyone or violating the ToS so they're not a problem. Live with them.
Serial inviters you can't do much about (although here the abuse team's decided to step in a bit). You don't need to do anything about them. The only thing they can do is mailbomb you by sending tons of invites, and the only way you can be added is by clicking on the link in the e-mail. Doing nothing will result in nothing happening. It's amazing how hard that concept can be to grasp.
And people wonder why spammers still exist.