To delete many Items at once, hover your mouse over the Manage button in the header menu.
A drop down list will appear.
By selecting the option "Items", a new page will open up with a list of all your company's Items.
Select the items you wish to delete.
On the top right hand side of your screen, to the left of the export button, is a smaller button marked with an ellipsis "..."
When you select the ellipsis "..." button, a drop down list will appear with a single option "delete selected".
Once you click this button, all the selected Items will be deleted.