that’s a bad idea. I have a lot of anti-spam measures implemented in my forum (regexes for suspicious e-mails and names , a local list of nasty e-mails , a daily updated local copy of StopForum’s banned IPs , querying the e-mail address at SFS  and BotScout , filling out the registration form in <2 seconds ) and keep a log-file of rejected registration attempts. Since mid-October last year I blocked ~4,200 attempts.
You don’t want to go there.
Bots for sure. On the average it takes them 3.3 milliseconds to register (0.35–16.8 ms). Breakdown of my log-file:
I’m considering to switch off everything excepting the timings. If a registration takes milliseconds, that’s a bot. Full stop.
Alfie (Helmut Schütz)