I don't own this lens but I do own the 15-85 which is prone to lens creep as well. I addressed and solved the problem with 2 O rings. I use a combination of a smaller diameter one and a larger diameter one. When engaged, it works like a lens lock so when I'm walking around, I have it locked and when I'm shooting on a tripod with the lens pointed down, I lock it in place after I set up the focus length I want. I keep the smaller one in place and rotate the larger one off and on. On my 15-85, it rolls onto to the manual focus ring.
I use the Danco Stock no 35705B (2-3/16 x 2 x 3/32 #89 O-ring) and the Culligan OR-233 O-ring. Both are available at my local Ace Hardware store but I know the Culligan O-rings are available lots of places since they are for a common water treatment product.