No I really can't stop all of the spammers, but I can slow them down quite a bit. Or at least make it so that only the most inventive and persistent can get through.
There are already controls in place to stop nearly all of the spam bots, but "real person" spammers can still get through. My programmer came up with a method that pretty much stops spam bots in their tracks. Such that only a REAL person sitting at a keyboard creating an account is able to set up an account here.
I go through periods where I will monitor new registrations closely, and then other times when things seem pretty calm, I forego the scrutiny. This has been one of the periods when I have been lax in guarding the gate. So today I resumed a little bit of the challenge to registration by requiring verification of the email address. This is also a challenge on this end, because some email services have a pretty lame brained spam check that will often reject emails from forum based websites. So valid members will sometimes get blocked simply because they are using gmail or yahoo (among others) to handle their email chores. Since some will get through and others won't I suspect that the users themselves are setting the spam blocking just a tad bit too high in the email options.
If the spammers still get through and get to be a real nuisance, I can go to full auditing of new memberships whereby I require email addresses to be verified and I manually verify the IP address to match the given location they provide. Generally speaking, there are a number of countries that it is a pretty sure bet that any registration originating from them are genuinely spammers, and they are rejecting immediately.
This can be somewhat of a manual chore here, but it's not too bad because new registrations has only been between 3 and 8 per day. On FaunaClassifieds, it is much more burdensome because of the higher number of new registrations every day.
And yes, some spammers are even clever enough to be able to get through any tests I or my programmer may set up. I have been seeing more and more spammers from out of the country actually leasing servers within the USA to launch their spam attacks from. So when I see that happening, I will simply block the IP address. In some instances, I will clock and entire group of IP addresses when I get three or more from that same group. In most cases, when I see ANY spammer register, I block the IP address anyway. But since blocks of IP addresses can be traded and moved around among hosts, I do have to clear this list every now again and start over.
So there are ways to block spammers. The trick is to not have any, or at least not much, collateral damage incurred in the effort. Well, that, and just not make the effort more of a burden than it is worth doing.