I've just had a look at the registration process and there's
1. a captcha
2. Question about capitals.
3. Another question about capitals.
4. A maths question
5. A question about GTIs.
6. Another maths question
7. Type in some text
TBH if they're putting the effort in to get past that, there's not much you can do to stop. Other than maybe having a waiting period eg 10 days before being able to post.