If youve tried everything, then thats it, theres basically nothing left you can do. Ive been there many times before, hating to 3/4 a cow!! I wouldnt advise putting dry cow tubes in the affected quarter- people do those sort of things and some how get away without an antibiotic failure. Best way is to shut the quarter down, but do it gradually- I put the milk in on 3, am, dump all 4 pm, for about a week ( it depends how much milk they are giving) then miss an extra milking on the quarter etc, so you gradually shut it down. If it basically dries itself off theres bout a 60% chance it will be ok next lactation. I find it will be ok to start with, but half probally reoccur during the next lactation at some time. Benefit of doing this is at least the milks in the tank, the cows not spreading the bacteria to others, drug costs, time treating, possible failures etc. Try and get a dry tube in when you dry the other 1/4s off. There is no benefit, so my vet says by using 2 or 3 tubes. Hope that helps.