I tried that module and removed it right away…....something similar happened…....
There are many ways to handle forum and other spammers, but IMHO this is not the right way…..you can dig around and find some other hints, but basically….
1. Approve each new member manually….if you look at certain info, like their IP, username and email, you can often sniff out that they are spammers.
2. Exclude IP blocks from areas of the world that you don’t serve in the blacklist, country thingy, etc.
3.Use the “member” utility to see if people populate their bios or URL’s with spam.
After that, you can usually rely on the member utilities:
http://www.purple-dogfish.co.uk/docs/member-utilities-1.x/index.html
That is for 1.x.
If you have 2, then you might have similar or other options.
The custom captcha is a neat trick - that is, ask a question that every real user of your site might know the answer to, but that some dude from Pakistan (or where ever you want to exclude) does not.