There are two ways to unassign an item in LaunchPad.
Method 1: Unassign using the "Assign" button
Navigate to the assignment you wish to unassignFrom the LaunchPad table of contents, under Assignments, hover your cursor over the activity you wish to unassign. Click Assign on the right.
Click the Unassign button at the bottom right of the window that pops up.
Select "Unassign" to confirm
A warning pop-up will appear alerting you that this action will remove the gradebook entry for the assigned item, including existing student grades. If you still want to unassign, click Unassign. Otherwise, click Cancel.