I don't think there is a way to unconsciously stop. Now you've noticed it, you'll have to keep an eye on it, and whenever you notice it happening, loosen up and try again.

It'll take a while, but eventually you'll be able to stop reminding yourself. As I say, I don't know of a magic answer for this - it's the same with any type of tension...

On the barre chords - are your fingers up nice and close behind the frets? That might help a bit... otherwise I'd suggest going back to basics - go slow and play one note at a time. Figure out where the buzz etc is and concentrate on that. I think everyone I know went through a period of playing where they held on so tight their fingertips went white. I'd suggest playing slow as I say, and seeing how much force is actually needed to fret the note.

Finally, does the buzz etc get worse / vanish at any point on the neck? just wondering if it could be the guitar rather than you...