From the screenshot, you can tell that "Storing" errands are set to Disallow. However, not only is it still appearing on the task list (with a mere 9.5 total priority), but the game considers it the highest priority task!
After about an hour of banging my head against the wall trying to figure out why my colony was cleaning itself to death, I finally figured out that the UI was lying to me. While the to do list is calculating the priority as if it were a Storage task, the actual game is calculating the priority as if it were a Supplying task.
Adjust a dupe's priorities so that supply errands are the top priority, and set storage errands to disallowed.