How can I stop the sales department from complaining to my boss every time I make a credit decision they don't like?

Answer: You can't stop them. More importantly, you wouldn't want to and don't need to. If your boss is does overriding your decisions very often and/or asking why you made certain decisions then he or she must agree with you. It is up to your boss to tell your salespeople to back off...and he or she will only do so when they have had enough.