What is the safest way to prevent fraud on GPT (Get-Paid-To) reward websites?

I manage a GPT-style rewards website (similar to HollaCash.com) where users earn points by completing surveys, offers, and app installs.

Despite basic protections, I still see issues like:

Multiple accounts per user

VPN/proxy abuse

Suspiciously fast completions

I want to balance fraud prevention without harming genuine users.

What techniques are commonly used in production GPT platforms to reduce abuse while keeping the system scalable?