On rubiks.com (where I have admin rights, but don't control the board setup), I asked the techies to set up a series of simple questions at signup. Simple things that a bot can't get past.
For here it could be things like "When using a computer, which bit do you normally look at? sound card, monitor or processor."
We use a few of these (I haven't checked since I'm not reregistering in case it throws a wobbler. It can be temperamental) and it works well. Since implementing, the only spam we've had is from people who registered before the new measures came in.
Most of the spam now is from old accounts that were set up in wait before the measures came in. Yesterday, I pruned all accounts with no posts that had no activity since the start of october. There were about 35000 I pruned!
i5 4440 3.1Ghz, Asus H87M-E motherboard, 8GB DDR3, 2GB DDR5 nVidia card, 2x500GB Seagate hd, Coolermaster 750W PSU running Mint 17.1 MATE and Win7
HP mini, 1.66 Ghz Intel atom processor, 2 GB DDR3, 320 GB hd, Mint 17 MATE