You should start handing out negative trust to anyone who doesn't pay on the agreed time, unless they ask for an extension first and another repayment date is agreed with them.
That should make them more wary of ghosting you.
Creating a flag and leaving negative feedback after 6 months since the defaulter last logged in the forum makes no sense. I've seen this a few times already.
If he takes it as a charity as I saw someone say once, let sashan consider the money as donated, although surely the one who received it was a degenerate gambler, so I wouldn't consider the donation as for a good cause.