Shelly it’s probably best to do what you are currently doing and review these on a case by case basis.
Where it’s obvious that the affiliate makes “satisfying the mimimum play through requirements” the primary focus of their campaign then I would suggest not paying them on a CPA basis and instead switching them to a net win arrangement.
Once finding profitable players is their primary focus both partners will benefit and I suspect that prize “give aways” wont be as attractive.