I had the same problem with my 18-200 but it can be fixed quite easily.
Go to any plumbers merchants and get a rubber O ring from a waste pipe fitting. You need to get one that is slightly smaller in diameter than the lens barrel so that it is a tight fit.
Slide this over the lens so that it sits over the join between the zoom ring and the fixed part of the lens. This will create some friction, and makes zooming a little stiffer, but puts and end to zoom creep.
I can now set my lens to any focal length and point it any any direction and it stays where I put it.
And no it was not my idea I read it on a website somewhere ages ago.
