Buy a Tamron, they have a lock switch fitted.

There's always one
Exactly where does the rubber band go?
On the barrel between the barrel and zoom section. It really isnt that obvious when on. It just provides a bit of tension at that point. It was an idea someone posted on another forum. I ordered a set from Australia on his say so. They are a matt black finish.
:shrug:
Here you go. Not the best piccie (N95), but you get the idea. Its only a simple silicone charity wrist band in black. I turned it inside out so it shows that matt finish on the outside. Its just about the right tension for zooming too.
Its nowhere near as obvious in reality. I just had to get close in to show it properly.
I'll have a look and see how many I've got. (I think its 3) If anyone wants one, pm me. First come basis.
Kev.