You can stop or pause the tracking of activities at any time in Memtime.
You can stop tracking at any time by right-clicking on the Memtime icon in your taskbar and selecting the "Stop Tracking" option.
If you are a macOS user, you will find this option in the applications bar at the top right of the screen:
To continue tracking, click the same icon again, and then select "Start tracking".
If you're interested in excluding a certain activity from Memtime's automatic tracking, you also have the option to blacklist it. Read more about it in this article here.