McDonalds is well within their rights to deny service to a customer (assuming it's not discrimination).

In fact I've seen them (and other restaurants) do this first hand.

A customer is being abusive, or giving the poor checkout person a hard time - if they get abusive or threatening, the manager will often intercede and tell them to leave the store.

Likewise, I used to work in a electronics store - if a customer is a being a jerk, you're well within rights to ask them to leave (politely). Obviously it's the nuclear option - but there's definitely precedent for it.