Suppressing dissent doesn't help your cause, because it doesn't change minds. The belief is still there, just suppressed.  

If you stay out the way of self-expression, everyone wins. You get to hear something that's dissonant to you, the other person gets to express him or herself. Having their point of view heard and accepted (if not agreed with), removes one obstacle to change. Maybe not all of them. And it's possible, just possible, having really listened to the politically incorrect thought, you might find your position shifts.  

Aside from that, believe it or not, it's not all about you. Other people have a right to speak even if you don't like what they're saying.  

Update: There's a difference between "dissent" and laying stinky turds in the middle of the room.