We have that problem once in a while with both of our long hairs. What I have done is either try to catch them and just wipe them off, or if it is beyond the just wiping stage, a quick wash off in the tub with some warm water works. They aren't happy but get over it quickly with a little bit of cuddling....also I found out that our vet with do a bottom shave and just cut the hair away from the "exit portal" and that works too. Just try not to snicker at them 'cause they look really silly with a bare butt....