I think user had no ill will, otherwise he'd at least change the name to make the original harder to find.
First of, issues like this should be tried to be resolved between missionmakers first, I'm pretty sure most people here are reasonable. If there are still problems, then contact a MMO.
Arma 2 missions -> message the original creator that you are porting their mission (no need to wait for permission from inactive user). If original creator for some reason contests, contact a MMO.
Arma 3 missions -> if one mission is clearly imitating another try to resolve it between each other (make sure it's not just a coincidence). If unable contact a MMO.