Or you can change the locks and throw all his **** outside
Everybody thinks they can do that (Throw all his **** outside) but you can't.
You cannot act with reckless regard towards anyone or their property.
He can sue you in Small Claims for the value of the property and he'll win.
First, pack up all of the things you value and move them to a safe place.
Make sure you have a copy of your lease with your name on it.
Then ask him to leave and give him a reasonable amount of time to find a place and pack his stuff and go. If you feel unsafe sleeping in the same apartment during that time, move out or get someone to come and couch-surf for the week to ten days you gave him.
If he refuses to leave or makes no sign that he is preparing to go, go to your nearest police department and speak with the Desk Sargent as to what the procedure should be to have him and his possessions removed from your premises. Your landlord (Management Company) should be made aware of what's going on. Hopefully you haven't violated the terms of your lease by having him there.
Follow whatever the procedure should is: Call the cops, report the squatter, ask that he be removed and that you or your representative be there as he packs.
It would be really good if he was to sign a document that says:
I ____________ have removed all of my belongings from __such and such address___ and have no further claims against ___Your NAME---
Dated witnessed by (Some Friend, some neighbor, some cop)
Most of the time these asses just go quietly, but be prepared.
Oh, and THEN Change the locks.