A tough situation — in order to solve a problem, your instinct tells you to confront it head-on. However, you cannot change the attitudes or maturity levels of strangers.
If these people have nothing better to do at dinner than make fun of others, take pity on their sorry lives. Ignore them. And move on, enjoying your life (in which your enjoyment does not come at the expense of others).
P.S. — If this happens again, simply ask the waiter to reseat you far away from the rude diners.