You can't remove the DLC (or choose not to install it) once you've purchased it. You can contact Steam support (as I noted in this answer) and ask them to remove items from your library, and they will generally do so. Do note that you'll never be able to get this back again without paying, however.
In most of my CoD experiences, the map pack playlists are separate from the base game playlists, so you may just be mistaken about not being able to play with people who don't have the packs. You should still have access to the base game playlists even when the map packs are installed.
Another route might be attempting to remove the DLC files from your machine. I don't have this game on Steam to test this theory, but I believe the map pack files are stored in two locations:
In your Steam folder, look for
steamapps - in this folder, there should be some *.ncf files that correspond to the map packs, for example:
steamapps\call of duty modern warfare 2 resurgence pack.ncf
You might try removing this file and seeing if the map pack no longer registers. There are also related files in:
steamapps\common\call of duty modern warfare 2\zone\dlc
I'd suggest backing any candidate files up by copying them to some other location before you delete the versions in steamapps. However, the worst case scenario is probably just that you have to run Steam's verify utility if you delete a file you shouldn't have.