You can't stop them, all you can do is make it harder for them - which will persuade some of them not to bother, and some to not do it so often. If you aren't careful, you will also persuade (or force) legitimate users to not sign up.


In addition to the things you are checking already, you could check the IP address that they are using to register.

This is not entirely reliable, as some ISP's give out a different IP address each time you connect to the internet (and then give your previous IP address to somebody else). There are also situations like schools and internet cafes, where it is expected that over time multiple people will re-use the same IP address.

So if you do check it, you should only block people if the IP address has been used multiple times (say 5 or more).


Another thing you could do is to make the registration process a bit harder, by adding extra CAPTCHA or email steps etc.

This is really only worth considering if you have people using programs to register, because the effect on manual registrations will affect all users - so you will probably lose lots of valid users too.