Low device limit per phone number/payment card, with the standard checks for VOIP would probably make things painful enough for most. Heck, outsource the bot ch

Low device limit per phone number/payment card, with the standard checks for VOIP would probably make things painful enough for most. Heck, outsource the bot checking and require a Facebook/Gmail/Apple/Twitter/whatever login. Intrusive as heck, but it works relatively well since those companies have already whacked a million moles.