How about a steadily rising penalty the more you quit. With a reset every 24 hours.

Quit once, standard 15 min penalty. Quit a second time within 24 hours of the previous, 30 min penalty. For every quit within that 24 hour period, the penalty increases by 15 PER quit. This way, it penalises players on a steady slope, rather than all out banning them from it for a really rough spell of quitting.

If i do a hard mode Ifrit, find that the group is incompetent after 5 wipes, i get a 15 min penalty. That is fine, i go make a coffee and a sammich and queue again. After 5 more attempts with a new group, i get really peeved at the total lack of coordination and quit. 30 min penalty now. Again, 30 mins is not terrible so i go do something else for a bit. Next group, i get another troll intentionally wiping the group and forcing me to quit again. 45 min penalty. Again, 45 mins is not really that bad. You cant really NOT find something to tide you over for 45 mins. In this example, for only 3 quits i only got a total of 45 min penalty which seems fair.