Some shower valves have built-in, or in-line, shutoff valves. If yours has
that feature, you will see them after you remove the big round chrome cover
that covers the hole in the wall.
The in-line shutoff valves look like two flathead set screws, one on each
side of the valve -- one for hot and one for cold.
Here is one model of the Mixet brand and the shutoff valve screw is shown as
part number 19 on the exploded view diagram:
http://www.plumbingsupply.com/mixet.html .
And, here is a YouTube video showing a similar setup on another brand:
http://www.youtube.com/watch?v=jnPNR6douzg .
On my shower valve, the screw-type shutoffs required turning them clockwise
all the way until each valve shut off. On the video, it shows a quarter
turn of the screw. I guess different brands use different systems -- if
they have the in-line screw-type shutoff valves.
If your shower valve doesn't have these, look for a main shutoff in your
house, or get a shutoff curb key tool at Home Depot etc.,
http://www.lowes.com/Search=curb+key+tool?storeId=10151&langId=-1&catalogId=10051&N=0&newSearch=true&Ntt=curb+key+tool#!
and turn the water off at the curb.