Here is an idea of using Custom Profile Fields to stop spambots registering.
Custom Profile fields can either be set up as a dropdown box, easier for users to use, or as a type-in field.
1. Find the Custom Profile Fields in your Admin Control Panel
2. Use a simple 'Are you a spambot' type question
3. Set the 'Profile type specific options' as :
Notice you are rejecting the default value here
