Just a derabanan? the cheese could be traif midoraisa if rennet from a traif animal is used in the production of the cheese. if any vegetables are used, there is the issue of tolaim, which is doraisa. as has been already raised, the pizza may have been baked in an oven with pepperoni, which is a)traif and b) bassar bichalav both doraisa.
Why do I think this entire thread is contrived?