What CPA allows content locking for "contest" sites?

Basically, here is the deal. The site will allow users to submit an entry to a contest that will giveaway something like a game, software, electronics, etc. The giveaway is actually real...so no blackhat involved. What I want though is for visitors to fill out a survey before they can enter the contest by using a basic content locker on the entry page. I am unsure of what CPA networks that are reputable would allow this so any help would be appreciated. I don't want to waste my time or the CPA's time if I can avoid that!

A lot of CPA networks would consider this hard incentive. While there may not be any blackhat involved, users will be tempted to cheat your contest by using proxies. While I can't speak for all CPA Network, AdGate Media won't accept this sort of traffic unless you have extensive fraud prevention in place.

I contacted a few who were willing to accept this, but I am curious of what anyone could do to prevent proxy fraud in the first place? I don't want to deal with it by any means, but I don't know how I could stop it though?