Captcha won't stop spammers from registering to my website

I suspect some of the spammers are working manually, and preventing signup spam is one of the toughest in my experience. In this case it is impossible to entirely prevent them from registering, and you may need to set registrations to require manual approval by a site administrator. I like to combine Captcha with Honeypot module against bots. For a more complex solution (which I have set up but not used in production) look at Bad Behaviour module.

Another approach is to check where the signups are coming from. If they are mainly from one or two countries you could ban people from those countries' IPs from signing up (or accessing the site at all).

Yes, I'm having that problem too. Hundreds of such registrations per day. I find it hard to believe that someone loves my web site and forum so much that they are hand-registering another bogus registration every 5 minutes, but that seems to be what is happening.

About sources:
mailinator.com
yandex.??
mail.ru
in fact, anything from Russia, Poland, or Romania is almost certainly spammers.
Unfortunately, I don't think you can block entire countries by IP number.