How do I stop people from owning shipments and printers when they use their gravity gun on them?

Ok, so I made a server and everything was going smoothly, but then someone on the server told me that if you try to pick up a shipment of guns you own the prop and then no one else can move it. Does anyone know how I can fix it so that if you pick it up you wont own the prop? Thank.
Rapidk1ll3rz