Recently, I have noticed that the "Recent Wiki Edits" box (equivalently, this page) in the sidebar seems to be almost exclusively filled with the edits from spam bots and either Gwern or Vladimir Nesov cleaning up after them (thanks!). This seems like it should be fixed, if only to save those two the time they spend on maintenance.
The wiki registration form does have a reCAPTCHA in an attempt to block spam-bots, but this is apparently not effective enough (maybe because reCAPTCHA has been cracked, or the spammers are using humans in some way).
I have some possible solutions, but I shall wait a bit before suggesting them.
(I vaguely remember there being a discussion like this previously, but I can't find it again, if it exists.)
You can also shorten the timeout on the captcha. A common captcha bypass method is to get the captcha for some site, use it as a captcha on some site you run, and use that response on the one you're trying to bypass.
One thing that can help is to disguise and indirect the captcha. Put a fake response field in with nearly zero screen size, with the name 'Captcha response', that an automatic captcha bypass could put its response into, and then have an 'inappropriately' named but properly placed field immediately after, that people will actually see.
The LW trivia is of course a better solution, but this one is much easier to implement - just a few static HTML fragments here and there.