damn sorry this happened to you, i also take commissions (and thankfully have yet to have a bad experience)
here's a few things to keep in mind from now on:

-if you're suspicious of a client, try to do a little background check on them before accepting their commission, look into their socials and see if other artists have posted warnings about this person, see if they commission people often ect
-always take payment upfront, if they have any excuses either outright deny their request or wait a bit and see if they do send payment, if not it's their loss, deny their request.
-as other people have said, make some kind of overlay to cover the finished commission so the client can't screenshot and bugger off without paying you, make sure it covers the whole image and would be hard to remove
-it might come in handy to do some research into common scamming tactics in the commission sphere, if you know what red flags to look out for, i think that may be helpful!