I am sorry but the guy is a thief (didn’t he steal from you?) user (didn’t he live off you?), an addict not seeking help? Etc etc etc People who steal and live off others do not usually listen to voice of reason.

Do you want to send him a message that he is still on your mind? Don’t think so. You already rejected him indirectly by posting on other forum that you are dating (no matter if you currently don’t, eventually will be again). So that’s a rejecting message right there. You moved on.

I’d focus on how to learn avoiding dating wrong men rather than how to make those wrong men understand something. This guy will not understand or care for what you have to say. By not contacting him you send him a message that he is of no importance and you are doing just fine.