If you have a ton of scheduled reports that you want to cancel, it's a big pain in the neck to do it one at a time, because you have to click 'Cancel' then click 'Confirm' then wait 5-6 seconds. It's not long enough to do something else, but just long enough to lose mental focus and feel like it's a big waste of time.
Here's a little trick that helped: when you get to the page full of reports you plan to cancel, copy and paste that URL into 3-4 browser tabs. Cancel and confirm on the first tab, then switch to the next tab and cancel/confirm another scheduled report while tab 1 refreshes...then switch to the third tab...rinse and repeat.
This tip might help you as well: even though your only dropdown options are to show up to 50 items per page, if you need more, that number appears at the end of the URL (it says 'item=50' or something) - you can change that to 100 or 150 or whatever you want. Hit enter and the Data Warehouse manager will display more items for you.
