Would it not be an idea to only give RP to verified or premium users? So anybody owning 2500 FUN tokens (which is affordable for almost all members) could get the full amount of RP per freeroll or from referrals. I doubt that a lot of bot users would buy the tokens, so that can be a way to reduce abuse.
You would be amazed how many have bought 2500 for many accounts just so they can bot without captcha. At today's prices that's staking $50 to steal $50 every 3 months. When I catch them they quite often try to justify it by calling that making an investment.
Damn, many are smarter than I thought lol. Other idea could be to only give RP to people that are already a longtime member of the site (i.e. if you are member longer than 1 year than you will be able to get RP ...).