I’m afraid simple mail activation isn’t much for people that really want to spam some forums. It’ll help for sure, but only for some time. Then next spam script that is a little bit more intelligent will spam forum if find it (spam bots can be really annoingly inteligent).
But when security done with few things simultaniusly it would be enough for bigger span time.
1) Recaptha on registration
2) Mail activation, no posting without activated account
3) Maybe even simple recaptcha on topic creation (only that with checkbox required)
4) As you are using WordPress and BBpress forums (one installed with ultimate member I think), you can easily throttle user interactions in wpadmin panel – throttle posts posting to 1-2 minutes.
5) I don’t remember now, but inside bb Akismet is built in by default, so needs just to activate it; this could help a little
6) Another way is to redirect every POST request to /register/ page that doesn’t have referrer from syngates.com
7) You can blacklist ip’s of spambots that already tried to spam here
The easiest and 5 minute way to deal with some of spam bots is to create honeypot for them. Silly ones won’t deal with empty hidden input field that should be empty. It’s really simple to check, as spambot will put something in there (it analyzes code so it “sees” it there) but it will be visually hidden from humans. Especially if it has label or something with “required” word around it. So just create empty hidden field and then check it. If it is empty it’s human if not it’s barely spam bot
Oh and there is plugin – Spam Destroyer that adds honeypot that I wrote above to forum forms
Maybe you and your team find in that message something that help you to deal with spam @syn