How do I exclude a song from iTunes Match
I don't think you can do this with a single library, iTunes match will check everything in your library, under the 25,000 track limit, to try and match.

One possible alternative would be a separate iTunes library for the files you don't want to match.

This separate library could also contain the actual files that have been matched by the other library as well but not be enabled for iTunes match itself.

You could then use this 'super' library as the main one running the iTunes match library occasionally just to keep iTunes match up to date.

How to run multiple libraries