Whose name is on the lease, and were the police called?
if they took off, they are no longer stealing. are you co tenants or do you act as the landlord? if you act as landlord, then you have to follow landlord laws
1. It has to be a secure area or they can claim theft by you.
2. YOU have to notify them to pick up by a certain date and allow them to get it within a week's time or 14 days. Only after that date can you dispose of it as abandoned property.
3. I highly suggest you change the locks, notify your landlord about the issue and give the landlord a key with the understanding that their property is no longer in the apartment, that they are not allowed in anymore.
what is the market value of the stuff? if it just rummage sale thrift store stuff, pitch it and deny it ever was there. how long have they been gone how far behind? are they on the lease?