I have this site set so that all new registrations send me a notice via email, which I do scrutinize. But let's face it, I just can't be here 24/7. Nor do I really want to set it up so that registration have to be approved prior to them being able to post. That tends to be perceived as being hostile to new members, and I'm sure some will be lost when they find I am out for the weekend and after a day or so, they are still not approved.
New registrations DO need to have their email address valid in order to get accepted, which does filter out a good bit of the spammers. But some will always get through.
Pesonally, I believe that with the people here trying to keep a lid on the spammers, they are weeded out pretty darn quickly. So no, it's not perfect, but I do believe that it is acceptable based on the amount of effort it would take, and the drawbacks considered, of trying to be closer to being perfect with this task.