Our lie detector isn't a polygraph, it's a woman. I don't know if you're familiar with Verity Willis, but she has a history of being able to catch even very powerful gods in a lie. Which isn't to say she's infallible or can't be tricked, but we have a statement from both angels - Lucifer and Castiel - that passed her test.
We can't outlaw possession as a rule, because there are multiple species here that use it to different degrees. Yes, there are consequences for possession against the will of the person being possessed. And, again, if we can prove it, we will do something about it.