I think your best option is to talk to them and tell them you're there, and that you're not leaving. Complain about having stuff stolen, without accusing anyone. Ask them to keep an eye out for anyone entering your property.
I like this approach. It is surprising sometimes how someone who has never been treated with any kind of respect responds when someone does.
Maybe figure out which one is the baddest looking one and offer him some kind of compensation for watching your stuff. Maybe buy them some beer and cigs or give them some cash for every week that something doesn't get stolen. It would give them some incentive to actually watch your stuff.
Neither one of those will probably work, but I was thinking of what Jesus would do...
