What can I do if my ex refuses to give me back his apartment key?

Judging by your prior posts...you ex is still on the lease with you. Unless all three of you (you, ex & landlord) have a signed release form....he does not have to return the key to you and has as much rights to the apartment as you do.

Changing the locks without him being officially removed from the lease, in writing, can land you in court for an illegal eviction.

Change the locks! Tell the landlord you must change the locks for your security, and be sure to give the landlord keys to the new locks.

You should have taken care of this as soon as he moved out, whether he returned the keys or not. He could have made spares without your knowledge. It is silly to risk your security over something as cheap and easy to deal with as changing locks on the doors.

Change the locks and give your landlady the new keys before you actually install them. All you need in most cases is a screwdriver to install new locks.

Go to home depot and buy a new lock. They're like $10 or so. Make a copy and give a key to your LL.

easy.

Change the locks

buy a new lock

Rekey, obviously.

get new locks.