This feature is available in Enterprise and Ultimate editions.
You can use the Member Planning page to remove workitems from a member's backlog and moves it back to the No Owners grid with a status of "Unassigned".
Select Team > Sprint Planning > Member Planning.
In the Members grid, click on the + symbol next to the member's name to see the list of workitems assigned to them.
Locate the workitem you want to unassign, and then double-click on a row to make it editable.
Place your cursor in the Owners field, and then click on the x next to each team member's name.