I made a fix for this, and I have had zero spam on my site since I implemented it (from 15-20 per day).
You need to do a quick edit of some of the core code. It uses the stopforumspam spammer database to check users which are signing up. Please note, this won't work for all sites, but should do in 90% of cases (all sites get different types of spam).
I have found a fix. My site went from 5-20 spam posts per day to 0, using the stopforumspam.com API. Basically, when a user tries to register, it checks their IP address against the stopforumspam.com database. In 99% of cases, scammers (at least for my site), were listed as scammers on this site. I wrote the following code to implement stopforumspam automatically. Sorry, it is a bit messy and isn't in plugin form, but it works. Only tested on Q2A 1.7 alpha, but it should work on 1.6.3 and lower too. Please tell me if it doesn't.
Go to qa-include/qa-page-register.php, under FTP in your site files.
Enter the following under this: if (empty($errors)) { Should be line 100-120 depending on Q2A version, just use CTRL+F.
$ipspam = explode(' ', $ipspam);
if ($ipspam[1] == 'yes'){
exit('This message is shown to scammers, change it to whatever you want');
}
Save it to the live site and you should be done. Create a test account just to make sure it still lets legit users through.
I got several upvotes and a selected answer, so it seems that it's working for everyone.