I get that as developers you are probably hurting in the ass that people skipped some of your hard work. But its because you made something terribly.

So you have 2 options.

1- Make the trash not a complete waste of time. As it stands their is no bigger waste of time for all players then the trash in dungeons at level 50. It offers 0 exp. 0 money next to no items (the ones that do drop are also pretty much garbage).
A good fix would be to make a low chance for them to drop real items like gear or potions or anything that a player could use to better themselves, or they could drop money even that would be better than nothing.

2- Wait for everyone to discover a new way to exploit/ give up on the trash game full of trash that wastes time. You have a big unexpected player base don't scare them off by being dicks and wasting their time.

I get that you are trying to delay players from experiencing what the game has to offer but this is not the way to do it.