If someone is behind me, while I'm working on my computer, & it ain't the boss, I'd stand right up, look the person in the eyes and say, "is there something I can help you with?" If they say, "no." I'd say, "please don't stand behind me, it gives me the creeps!"
(a**hole under my breath)
As far as someone touching your stuff, I'd not hesitate to say, "I'd really appreciate it, if you didn't touch my **** ! (Or stuff)
That's just me.
*big warm friendship hug*