I have found that when I personally ignore those instincts and hang out with the person, something goes very very wrong in the end.
I have a guy like this who comes into my shop once a week or so and gradually I can see he is trying to create some sort of friendship. He just oozes badness, he hasn't actually done anything, just his eyes and facial expressions don't add up. I try to be as pleasant as possible as I do with all my customers but im 110% sure this guy is a real wrong'un. Cant avoid him really so I just try to limit the amount of contact. It's actually quite interesting dealing with the public as you notice the different feelings or energies people give off.