We have anti-hacker software that checks incoming email addresses and IPs against a known hacker / spammer database. So we're already blocking quite a lot of people. The only ones that get through are the ones coming from "clean" systems. So if anything it's a sign of just how many millions of people out there now do this sort of thing on a daily basis.
There's no way to know, with a freshly made email address on gmail, what someone is going to do with it. So until they make that first spam post somewhere on the web, they can get in here. I then report them to the anti-spam group so they can't cause trouble anywhere else.
I do block brand new accounts from sending PMs or posting URLs, so they can't do *too* much harm. But, yes, the posts do still need to be deleted.