If you haven't yet, you could start with:
Enable Email Verification (Control Panel » Registration Settings » Enable Email Verification?)
If that doesn't help you could:
Approve members before they're allowed to post (Control Panel » Registration Settings » Enable Registration Queue?)
When version 7.1 is released you could enable visual CAPTCHA
I'd also put all the spammers IP addresses in the ban list (Control Panel » Member Management » Ban Lists), as well as ban suspicious domains they're using in their email addresses.
These things should help a bit.
