A couple of forums I was member/moderator of, there were two helpful features:
Moderators could post a warning (usually on topic expected to get out of hand) that posters had to click and acknowledge before commenting. Not on each and every post, but each time re-logging to website and wishing to post on topic. That cuts the 'i didn't know', 'I didn't see' excuses.
Similar but personal - warning pm that basically blocks all further action on website until acknowledged. Poster is not suspended, banned or anything, but can't ignore the warning.
Most times, the first measure was good enough to tone things down some, and we were able to set it to appear automatically every X posts, at the start of each page and so on.