With difficulty I'm afraid.
The most popular method is to move those tracks which you don't want in the cloud to another iTunes library. This does have a couple of issues though. Unless you have a separate computer which you don't want signed up to match then accessing these songs is a pain (you would have to swap between libraries). Also, if you still sync with another device by USB then these songs won't be accessable.
I do this, but use another computer which is always on. That way I can still listen to these songs via home sharing. It is still a bit of a pain.
Another method also involves having a second iTunes library, which needs to be signed up to match but doesn't need any local content. This could be on a different computer or it could be an alternative library on your main computer.
After signing that library up with match, find those songs which you want excluded from match and delete them from the library, making sure that you say that you do want them deleted from the cloud.
This will remove them from the cloud, but doesn't delete them from your main library. They have an iCloud status of "removed" and will not be added back to the cloud.
There are a few things to beware of though.
Firstly, all these songs have to be added to the cloud first, and you can't exceed the 25,000 limit at any time. You would therefore have to go through this procedure quite a few times.
Also, if you sign out of match for any reason then, when you sign back in again, it will try to add these files back (or more likely refuse to do anything because of the limit). You would then have to delete them and start again.
For that reason, I wouldn't recommend this method if you have a large number of tracks you want to exclude. Accidently clicking turn off match could destroy hours of work. I have done this soley for the Beatles Mono albums, which I want to be able to sync to my Nano for the car, but don't want in the cloud.