If you mean the Paypal button on your shopping cart page, there's a switch for that in the Express configuration, though it may only be there in Zencart version 1.5.X. Worth a look. If that option isn't there in your version, you could add something like this to your stylesheet:

#shoppingCartDefault #PPECbutton {
display: none;
}

We can't actually test the problem without taking a chance of buying something, so we can't duplicate the problem.