Do you do much hiking alone? If someone gives you a bad vibe, forget manners, just say no thanks and get away fast.
As far as a hiking partner that ends up becoming a pain, that reminds me of the Mary Ellen character in "A Walk in the Woods" , if you haven't read it, you should. To get rid of their pain, they hauled butt and got ahead of her. That's one alternative, high tail it til you're sure you've gotten rid of them.