Integrated captcha?

Hello,

     We're having a problem lately where spammers are using our subscription forms to mass email 
subscription requests. Its pointless, since the subscriptions contain no messages from the spammer - 
but its happening anyway. The end result is delivery failures for tons of tokens (an annoyance to the list 
admins) and the occasional blacklist of our IP from less intelligent mail services (I'm looking at you 
Frontgate!). 

     A simple solution would seem to be integrating captcha into the subscription page. It seems likely 
that this might be something someone has already done? If anyone has any code to share I would 
appreciate it.

    If anyone has alternate suggestions to solve this problem that would also be appreciated!

Thanks,
Alex