You can hide the webstore but not block it as far as I know. They can still find ways to navigate to it.
If it's proxy or game type plugins you're worried about, I'd try relying more on your web filtering solution and less on the clients. You also might want to consider blocking unknown sites for students if you don't already, that way the new stuff that comes out is typically blocked until your web filter categorizes it. We've had pretty good luck with iBoss this year.
