Kind of a hard stop.. but I could imagine making it so all loot is distributed at the end of a dungeon or once the timer is up. It might encourage them to finish the dungeon with you (or wait the whole hour and a half).

It would also stop farming the first few chests of a dungeon and then rejoining which, imo, that farm strategy seems kind of silly and unintended. Though that'd be something SE would have to decide if intended or not. For normal groups it might encourage people from leaving as easily as well (may say "one more" one more time than usual lol).