Last week I did reach Cooper mid afternoon. A monohiull was on my ball. I pulled up behind them and asked "Excuse me, what number ball is that?" Followed by "Do you have a reservation for that ball"(I reverified my email confirmation first). They acknowledged they did not have a reservation and promptly departed. I pointed out there was one remaining FCFS ball (but likely not very comfortable for a monohull in that location).

I believe it is helpful to inquire in a less threatening manner (vs "You are on MY Ball! GET OFF!").

This has happened several times over past several years (I am there a lot), and the offender has released without an attitude.