If you need a break from the interruptions, you can "Pause" StretchClock, and then you can un-pause it later:
1) Notice StretchClock in your Toolbar.
2) Check out your options including PAUSE
HOW TO COMPLETELY REMOVE STRETCHCLOCK for Firefox
1) Open Firefox.
2) Click the Menu button in your toolbar (or select your "Tools" menu)
3) Click "Add - ons"
4) From the add-ons window, click "Extensions"
5) Click "Remove" in the StretchClock section.
Following the steps above will stop all stretch breaks and will remove all of the files that were installed.