It would seem well within anyone's rights to refuse.
RAT5, in short you find that you can indeed refuse, but that suddently you have very little work to do and that others are called in preference. As a well known middle management figure at STN has said, "we can sort out the problem contractors by not telephoning them". This technique has become an art form with the cabin crew. It quickly brings those with debts to heel, especially when colleagues explain exactly what is going on.