First thing first, in situations such as bosses in dungeons, a player greeding only costs a healing resource from the healer, the boss turning around could lead to multiple players dying from it.
Telling them to leave the boss earlier is definitely easier than telling them to use Elusive Jump on cooldown as an example. Don't forget that if they weren't using their tools properly, then you would also have to explain to them why and how, which for the greed you can simply say "there is mechanic, stop doing your rotation and get to where you have to be"