I doubt you can ever be fully stop bots. The only way I can see to significantly reduce bot is to make everyone pay a one off £1 to sign up and force the use of a debit/credit card, no paypal, etc. The obvious issues are, it removes annonimity, and blocks entry.

Possible mitigations;

• Maybe you don't need to keep the card information after the user pays for sign up?
• Signed up users can be given a few "invite codes" a year enable those who don't have the means to pay the £1 to get an account.