You can add a "Clear Playing Now" button to the toolbar in MC. It is a standard function already available.
I don't have JRemote, so can't comment on whether it has a similar feature, but I would be surprised if it didn't.
Of course, some settings allow you to replace the Playing Now list, rather than adding to it. Replacing items in Playing Now would be just like deleting everything in Playing Now, and then adding something new.
If you are talking about actual Playlists rather than Playing Now, well, just delete the whole Playlist and when you want, create a new one.
If I have totally misunderstood, explain why and when you need to clear which Playlist.
