Please stay in my coffee shop.

Although that person is using your internet with a $4 coffee, they're occupying space after everybody "is gone"(9am-11am and 1pm-4pm), contributing with their physical presence.

A newcomer, that may end up becoming a long-term customer, would feel more biased towards places with more customers( or the idea of).

So, although it might seem financially reasonable to kick them out, I doubt it's the best answer.