If you are able to ban IP's you can put an IP section in the form, so when they send out email's or spam it to you, take the first two setts of letters from their IP, and Deny the IP.

Easy as cake. :P

Well, change the settings of the form.
Maybe $email change to $EmaILL =
or
name="ETllD"> and bots won't be able to read them as well. :)

I hope you can understand these options :s
I've never had a spam bot, and be able to use my stuff.