If your site was in the top 1,000, you'd have a different experience since the spammers would be adapting to every one of your exotic captchas no matter how many times you change it.
1. fofou.org hosts the forum for my SumatraPDF site, which is 33,279 in Alexa. I think that's enough of a target, more than most forums out there.
2. I don't think it's productive to bring top 1000 websites into this particular discussion. OP's forum wasn't that popular and most forums, by definition, won't be either.
Once you're there you're playing a completely different game and at that level you should have resources to win it. I don't see spam on Wikipedia, StackExchange, Quora, HN etc. When there's will, there's a way to fight spam.
Your software solutions can do only so much. Wikipedia, StackExchange etc has Human monitoring (volunteers who report spam) in addition to automated stuff.
Its tough keeping up with spam.
P.S. Thank you for SumatraPDF!
If you have showdead turned on you see a variety of obvious spam (handbags and shoes) and non-obvious spam.
And there have been some aggressive voting rings.
No matter how large your site, if you give people you trust the tools to stomp out spam/bad content, they will use it and you eventually "win."