Avoid parmesan and inform restaurants that offer it as a vegetarian option.Anyone familiar with the blank look on a waiter's face when asked if their pesto is vegetarian ("yes, pesto is just basil and pine nuts", "and parmesan?", "oh yes, and parmesan, but that's just cheese") will know that sometimes it is easier to avoid such dishes. Or, where parmesan's presence is not stated on the menu, hastily push any offending shavings to the side of your plate before they start to melt.
As it is, the law states that cheese is only allowed to be labelled Parmigiano Reggiano or parmesan (the widely used French name) if it meets a number of criteria, including being made using calf rennet. It's nothing new; parmesan has had Protected Designation of Origin status since 1996.