If you go to the matches "home page" where it shows who is facing who for the particular match you will see a bunch of comments at the bottom. Scroll to the very end of the comments and you should see the following type of box. Make sure it is unchecked.
If this doesn't work send me a link to the match and will try something else.
I was registered for a team match and unregistered when it became clear it was not going to start for several more weeks. I'm still getting comment alerts for the match forum, but I no longer have the ability to comment in or untrack from the forum. Anyone know how I can stop receiving alerts for new comments in that match?