Just guessing here, but who-ever it was who dropped the bike off covered in shit, probably thought they were just taking their bike to any old garage & were totally unaware of your extremely high standards of work.
Although one look round your workshop should've informed them of that & they may have realised they had made a mistake. I'm sure as someone has already said you let them know about it.
If it was someone who was aware of the standards you work to, then just take it down the end of the yard, chuck a gallon of Gunk all over it & let the rain do the rest, or even better jet wash the f****r. Probably need a full rebuild & paint job by that point.
Nothing worse than being taken for granted, sorry you have to put up with shit like that.