Spam user accounts being created
Mollom tests content for spam. But user creation form does not provide any opportunity to provide spam content in the first place. So you have 2 strategies available:


Protect user creation form with more traditional captcha, or one of it's mutations like reCaptcha or little game humanity test.
Protect comments / node forms with Mollom and automatically block users that will fail mollom test there.


Note, above ways are not mutually exclusive.