My first question is does the barn owner know someone is swiping your stuff? That is not something I'd want going on in my barn. Cameras are the only sure way I can think of to know exactly who is taking your stuff.
I lock all my stuff up at the barn I board at. No one can get into my stuff without my key. Where is your stuff when it's getting taken? If it's in a tack locker or something I've been known to set things up strangely(brush out of place, towel covering things, bridle looped around something) so that if anyone goes in there and it's different I know someone is messing around with my stuff. The trick is then figuring out who. Or tying bailing twine or something around the handle. Make it difficult for anyone to take your stuff, if they're really determined and try to struggle with it it will make it more obvious when they go to take something. Do you have anyone you suspect, or any one at the barn who could help keep an eye out for you? Locking things up if possible would be the easiest thing, though catching whoever it is would be fantastic. Stealing like that just isn't acceptable.
<3 Dallas, Dakota, and Shaymus <3
RIP Shaymus 8/16/13