Even before the reset, no one really cared for Koins either.
In their farewell thread, they should be forced to include a "return date" and it would have to be a date, not an estimated date(they wouldn't be able to say they'll "probably be returning within a month" or something stupid like that). If they want to return before their return date, they need to make sure it's alright with staff, meaning they'll have to post in private inquiries to return before their return date. If they come back before that and don't have permission, they should be banned until their return date.
Personally if someone makes a thread, but comes back the next few days as you stated, just temp ban them for a day or two or ban them until when there meant to come back just to stop them from coming back, either way resetting there koins won't do anything apart from piss them off since they earned it.
A warning level or a temp ban until the date stated on there thread should be a good reasonable punishment.
People still do this all the time (assuming for attention). Philly made a warning about this, but people are avoiding that or unaware that the warning exists. So I do believe their Koins or something should be reset.
But since there was a recent Koin reset, that's not a huge threat. Maybe a 10-20% warning for lying about leaving for [said time].