How do I prevent OS X from automatically installing app files on my drive
The Apps that the Finder shows in the context menu "Open with..." are not determined by Spotlight.
Independently of Spotlight the System scans for all .app files on your disk and builds a database of Apps and the file types they can open.
As long as you have an App on your disk, the Finder will offer to open files with it.

AFAIK, you can only prevent this behavior by either moving your apps to another disk, deleting them entirely or (what I often do) zipping them.

(By the way, OS X is not installing the Apps. Usually an App is 'installed' when it just is on your disk. Installing like on Windows is hardly ever necessary for an App to run on OS X)