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

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.