Cancelling a match means first and foremost cancelling the possibility of chatting with the matched profile. Depending on your wishes, it is possible that you no longer wish to chat with a person. 

In this case, go to the chat with this person and click on the three dots in the upper right corner and then click on "Delete this match".

You can also delete a discussion by clicking on a person's profile, at the very bottom of their profile click on "Delete Match".