WHAT TO DO IF I GET SCAMMED?

Ok so I made a post on here about someone training one of my accounts in Old School RuneScape.. He seemed a genuine guy and had over 1900 vouches. I Specifically asked him to get me no hitpoint strength experience by way of doing shayzien gangs.

So we agreed a term in discord chat and I paid him prior to him going on the account, he then logged into my account and got 14 defence and cleared my bank, he then laughed at me for doing it.. I have screenshots of the chat and would like to take this further, as a trusted seller he should not be able to get away with this..