Stopping GravityForm spam from real people?

There are countless numbers of plugins and methods out there to prevent spam from bots (Usually by just creating an invisible form and if it gets filled out, you know it’s from a bot) but my spam isn’t coming from bots, I don’t think.

My thought is that some company is hiring some workers to manually go to sites and fill out my Contact-Us form. I can’t do anything with IP’s because the entries come from all over. I’d imagine they’re using many proxies/VPNs.

I’m at a bit of a loss here, the spam is pretty relentless. I was thinking that setting up some kind of blocked-words list would help. Maybe even just decline any form that contains URL’s, broken or not (I’m sure they’d try h t t p : / / e t c).

I’ll need to deploy a solution to multiple WordPress sites, so doing anything custom and time-consuming wouldn’t be optimal whatsoever.

Any ideas or pointers would be greatly appreciated.

Using:
WordPress 3.9.1
GravityForms 1.8.9