Material replace

I’m creating a site model and would like to insert a few verticals to show some perspective. I have downloaded a few from Blend Swap, fantastic models with realistic material finish.
My problem is, I would like to show these models in a gray scale so that they don’t stand out from the rest of the scene.
Is there an easy way to strip all the materials from a model and replace with one material?

There should be an option in the Render panel that you can specify a render material. This will override all others.

That would be great and easy if it was just tick a box for the object to show gray!
Just need to find it!

if you simply want to remove the materials, go to the materials panel of the object and remove them.

you can also easily set a replacement material for all objects in a scene:

Thanks for your help