Remind them of the obligation to pay in firm tones, without minimizing. If the loan is secured by collateral, repossess the item. If you have it in writing, then you can sue in small claims court.
If the person is still not giving the money to you, go and give him a warning that you will report him to the Police. If he still doesn't respond, report him, provided you have the proof that you gave him the money.
