Not if you have higher status to begin with. Not if you are their friends to begin with. And if you merely have power over them, they will notice that their beliefs will attract the contempt of people who have power over them. Of course, this probably won't work on public web forums. Of course, this won't work on your boss. Of course, this will not work well on strangers.

Then again, the best way to do it is to act as if you were ridiculing a third party. If they get offended, try not to notice, and direct your condescension at those other people you're pretending they're talking about. Give them a way to save face.

Whatever the means, the idea is to stress that epistemic relativism is not socially acceptable. Make them feel that thinking such thoughts has consequences.