Introduce (or enable) the 'Report Abuse' flagging (that exists in the forums) in the comments. A click on the 'Report Abuse' would flag it for review by a moderator; if the moderator agrees, the comment could be deleted, if the moderator thinks the comment is valid and not abusive, then the 'Report Abuse' flag/link could be disabled/hidden on that comment. The 'Report Abuse' flag/link could also be disabled/hidden after a time period or number of views, to prevent abuse of the abuse reporting system.
This could be extended so that comment posters who have had too many postings rejected as abusive would automatically have the comments flagged for review, possibly before they become visible; too many more being rejected and their comments could be silently dropped.
I expect that spammers will find and exploit the comments mechanism for spamming, so an auto-trigger of hold-for-review in the case of multiple posts by a new user could reduce the impact of that; what happens to the comments if a user is deleted from the system?
A further suggestion: