L.!.
NUMBER ONE: MAKE A POLICE REPORT. It doesn't matter if he can't be charged. Get it documented. If he does something like this again or you experience harassment, presto! You have a police report of this incident, and whatever he does next will not be his 1st offense. Besides, if you find something of value is missing, you'll need the police report for insurance claims or criminal claims.
What was his relator doing while he was going through your drawers?! Relators don't let you go through other people's things... opening closets, looking in the pantry is alright, but no professional relator will let clients go through drawers or take things. Was he left alone in the house? Where was the relator.
You can ask your relator about banning that relator from showing your property. You can also probably place a complaint against the relator for unprofessional conduct, if she/he let your ex over-step the bounds. Either through the real estate agency that employs her/him or through the national association of real estate professionals. You'll have to do some research and maybe ask a your relator if/how to do it. The relator might have to carry insurance policy and perhaps you can make her/him reimburse you for any expenses related to changing the locks or the value of lost property.
As for your ex, the only recourse would be to place a restraining order against him.
And again, CHANGE ALL LOCKS. You don't know if this relator let him take a spare key or make a key off the one left in the relator box-?! Creepy!!