The best way to tell someone that they are boring!

I am confused. Why would you even need to do this?

I would suggest nonverbal language as they can speak louder than actual words themselves. When someone is talking, face your torso away from them, "sink" your eyebrows so that the light in your eyes disappear, slump a bit, sigh, etc. However, don't over do it as it may seem as if you're blatantly trying to be rude. And, don't do it so long, then settle for, "What you've been talking about/doing doesn't interest me much" if all else fails or keep making suggestions of topics and activities that oppose their boring ways. They'll catch on to the contradictions and really take the hint.