To prevent abusing it would be pretty simple. Lose the leve, but you are able to restart right away when you come back. However, you start at the exact point you were.

If you had killed 3/6 monsters, you would start again at 3/6. Also, only players that left the leve in the exact same point as you can enter your leve again (so, if you crash and everyone else finish the leve, they won't be able to join you this second time).