Is there a way to remove all items from your shop at once? Or even better, all items in a specific category in your shop? Thanks

Nope, there isn't. You have to sort them out by hand. Use the drop down menu and then after finishing one page "process prices and removals". That's faster than doing "remove all" for every item. If you want to empty a specific category you can at least use the category filter to have only those items showed :)