We agree that deleting multiple activities is cumbersome, which is why we added the option to delete everything from a given hour or day from the context menu. (windowscentral.com)
I get around 600 emails a day, and often don't read them all, so it's useful to just delete everything within a few seconds. (goodereader.com)
Finally, you can simply delete everything on a specific layer. (goodereader.com)