I have tried the "powerless group" method but find it quickly becomes a pain to deal with all the spammers that build up in there. On a very low key WWW site I found I was getting 10~20/day signing up and it was proving quite hard to discriminate who was good/bad. In short, while it's technically a solution that works it practically becomes very annoying.
May I suggest that in addition with what stapleton has suggested (the 6-step process) that the applicant needs to write a small note or message to the admin to validate that they are actually legitimate (perhaps saying why they wish to join or even just telling you what the forum is about 
).