This is a real problem for bookshops and customers. Some people come in several times a week read for free and even bring in lunch. Often the magazines and newspapers are left in a state which is no longer fit for sale.
So what can we do?
In my opinion those people are making things worse for the shop and all decent customers, so just kick them!
But after years of "the customer is always right, many people have become so arogant that demanding acceptable behaviour from your customers will definitly kill your shop.