Tell me about it!
Captcha doesn't work in its "normal" form. What did work for a couple of years was a non-standard polkadot "type what you see" - a bit like the Ishihara plates. They've now sussed that.
What I've done now is to add a couple of non-standard fields to the registration screen (easy to do via the ACP) and made them mandatory. That seems to slow down the rate of spam registration. If the entries in those fields aren't valid, I know to delete the account forthwith. I still have to look at them, though.
I also ban any IP range that offends - initially the first two blocks (eg 92.138.*.*) but eventually just the first block if it continues. I seem to have banned Russia and China pretty much in total, but that technique doesn't work well with botnets.
There's mounds of stuff about this on the phpBB Forum.
I'm also getting far more spam into the Spamcop junkbox these past few weeks - it's up to 12 an hour during the Russian daytime now. The daft thing is that it's the same half dozen spams most of the time. That plonker advertising for west-europe-consulting must write to his victims every half hour.