There are a few ways, but I can't remember my own registration process and what it involved.

Firstly, make it so that the admin has to verify each sign-up... but this can put people off as sometimes it can take days if the admin isn't around.

Secondly, on sign-up you can make it so you have to spell out a word that is drawn in among a bunch of mess (as I'm sure you've came across again).