[mtmail]

You could try a time-wasting count-down after the captcha. The "send" button can only be clicked after 60 seconds passed and you check if the IP matches to make sure somebody is not using multiple tabs in their browser. Still, it seems those spammers are motivated and have time (or found somebody to outsource cheaply to).

[rakjosh]

IP is not something I can rely on, because looks like they have enough unique IP addresses to use. And like you said, motivated and a lot of time on their hand. I feel like adding that 60 seconds delay only make the UX bad for genuine users.