Docking XP will only effect people who are looking to level up, there are many who are already at high levels and have all the equipment they want/need so will not be impacted.
I think the 24hr ban would be great. Anyone who wants to do something about the serial quitters would happily accept the occasional 24hr ban if that upped the completion rate for MP games from it current poor level to something more acceptable and less frustrating.
There could also be a system where the 'offense' stays on your record for say 1 week. If you quit again in that time the ban becomes 48hr and so on.
I am firmly in the 'something could and should be done' camp on this issue. Won't be so bold as to claim to have the perfect answer myself though.