Did you have this in your contract? Do you have a contract?!
If you aren't okay with someone, put your foot down. You WILL be held responsible since they're on your private property. If you make them all sign releases that THEY are responsible for their own choices and horses, you'll be in the clear.
All about liability insurance. Make sure you have it!
I know very limited information--there is more out there and there are more things you can do or things that you aren't responsible for. Research!
"Strength is the ability to use a muscle without tension"