Label everything and if it doesn't need to be there, don't leave it around.
Your feed, buy locking containers and lock them.
Your hay...leave enough for a few feedings and not more.
Make it your business to be there and be seen...be courteous but non-drama.
Keep your shavings there for "a" weeks use, nothing more.
People have a habit sometimes of helping themself when they run short, at your expense.
Supplements, put them in disposable baggies and again enough for a week if someone else is feeding...
Get a locking tack trunk and keep all inside it under lock & key!
The only "tack" you should need to have out is a halter and shank, blanket that is what the current season dictates... keep it neat and clean without the excessive clutter some have around their stall of everything just left hanging... a great place for bird poop, dust, dirt and grime to ruin your equipment.
If you don't want anyone helping themself to your equipment to try or feed to feed, make it unavailable for them to get their hands on.