Excellent article on forum/web spam and measures you can take against bots: https://kevv.net/you-probably-dont-need-recaptcha/
I've had to deal with this problem several times over the past decade as well. The article misses a very effective approach (if you can afford it): Disallow indexing of your site by Google/Bing/etc. Spammers use search engines to find sites to spam and their goal is usually SEO, so unindexed sites can't aren't even interesting to them.
(I'd send an email to the author, but apparently his only mode of communication is... Twitter)
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