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.
To continue tracking, click the same icon again, and then click "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 how to blacklist any activities that you don't want to track, in this article here.