Yes, I do actually. I mock them, I tease them. If I really don't like them, then I laugh at what they believe and start to formulate all these ways that I can hurt them and put them in pain. Not like I'm ever going to do it; I just feel it's a good way to get bad thoughts out of your head when you're mad.
Most often if I laugh at someone it's open and to their face. The only time laugh at someone behind their back is when it's an out of the ordinary interaction with someone I don't know, like a walmart cashier or something.
Some people may actually prefer you to laugh at them (or criticise them) behind their backs. American poet, Ogden Nash, said that backstabbers weren't such bad people because "a backstabber only stabs you behind your back -- but a frontstabber will backstab you to your face!"