Don't actually disagree (any more) and wind up that thread of conversation in favor of something else.
There could two (or more) possibilities as to whether the person wants to keep talking.
He may think you do not understand his point well (or well enough). If you haven't already, simply say you understand. Or take all doubt away, and summarize what he is trying to say. You can do this without indicating at all whether you agree with it or not. You are simply stating or proving you understand their point.
Secondly, they may think that continued discussion might change your mind. You can manage this by not being the second person in the conversation. Stay silent on this thread of conversation (or wind it down). You can change the subject. You may elect to simply excuse yourself because you have something to do (like go back to work, go get a drink, go to the bathroom).
In my opinion, the key is to tactfully conclude the exchange or simply to stop being one of the parties to the conversation.