I'm also of the direct school, or, rather: just plain blunt.

The first time he would have picked up something and it bothered me, I would have said: "Would you mind not touching the art work?" (or, in case it's unclear whether he'll understand what "art work" encompasses: "Would you mind not touching what's on the shelves?"

Or: "Hey, a tall guy like you, surely you can stick your head close enough to the sculpture without having to pick it up, do you?"

If I saw some level of offense I might add, as suggested above: "It's not about you, but I'm a bit obsessive about this... I had a [friend, brother-in-law, friend's husband...] break a super rare [insert fancy name of some art work here] and it could not be repaired, so now I'm really obsessive about them." Add, for fun: "My shrink is dealing with this issue with me but in the meantime please don't touch or I'll send you the bill of my analyst!..."