There are two ways I think you can combat this, neither is great, but either should work.
(1) When a player decides to join the paying league, you run statistics, note how many points that player has already earned, and then adjust the player's points in the first tournament they play after becoming a paying member. For example, say I've been playing your free rolls and have earned 75 points. Just before playing in a tournament today I decide to become a paying member. That means only points earned from today on should count. So, for today's tournament, edit Corey on the Players tab and adjust my points by "-75" (negative 75). That effectively zeros out my points going forward. If I earn 20 points today, Stats should reflect that I have 20 points, and not 95 points.
(2) Create a new player for Corey in the database and start using that player. There will now be 2 Corey's in the database, one for free rolls and one who is a league member. You could always delete the first Corey from the database if you want. That would even eliminate that player from showing up in Stats at all.