If you need to delete multiple activities in bulk, you'll want to do so from your Dashboard Calendar. Just select the calendar items you'd like to delete by placing checks in the boxes to the left of each of them:
Then, right-click in the row for any of the items you've selected and you'll see the below options popup:
Click Delete Activity. You'll then be prompted to type the word DELETE in a new pop — this is just a precautionary measure to help ensure that you don't bulk delete calendar items if you hadn't in fact meant to select that option:
After typing "delete" and clicking Confirm Delete, those activities will be deleted from your Calendar.