It's probably manual form spam and it's difficult to eliminate. There are some client side scripting solutions that might help get rid of some more automated and semi-automated spam but it sounds like you may have already tried them. The thing is that it's easy and cheap for someone to hire a person to enter this spam. Usually it can be done for about $5 for 100 or so form submissions. The spammer gives their 'employee' a list of sites and what to paste in and they go at it.
Client side timer scripts offer the best protection against manual cut-and-paste and other quick submission techniques. For example, a script that requires a visitor be on a page 1 minute before the submit button is activated.