you could try adding a "honeypot" field that is hidden to regular visitors, using something like "date of birth", which the bots will faithfully inserta value into. Then in your function you can discard anything which has a value in that field. I know its not exactly what you were asking but it could be a relatively easy win.

Also the hook you are using seems to be to validate extra custom fields in the form and return errors. If the bots are still getting through maybe they are using a different vector