Call the B/O and tell her what happened.
Ask her if she wants to deal with it or you will...then do use her answer as your guide.
In the meantime, if he is silly enough to raise the topic, I would not avoid him nor the subject, but make sure he knows you know he had your horse out and was messing with him regardless of what he was doing, he should not of been, period.
Jut state the fact he doesn't have your permission to handle your horse, never did and never will...hands off.
As of now, the B/O has been advised of your wishes.
Being spoken to by the B/O may be enough to stop whatever he has been doing.
Or it may not...
You've lost faith and trust though in the facility for safeguarding your horse so it is time for you to go, leave and find another barn ASAP.
Once trust is lost, you look at everything with crossed eyes and mistrust...
Find that new place sooner, like maybe by end of the week and start the new month in a new barn.
You don't need the grief nor high blood pressure from stress this is already plaguing you with.
Or, if the B/O really steps up and tells him to get out...my faith in the B/O and facility would be balanced again since B/O stepped up and removed the problem...
If he stays you go...if he goes you stay.