Can you requisition be reassigned to "Unassigned" in the Workbench after it was previously assigned

When I try to reassign it I only have the ability to select only another buyers name.  We would to unassign it so when we return the requisition to the preparer it doesn't have the buyers name on it.  If the preparer resubmits the req and after approval it does not go directly to the original buyers workload.  Please advise if it is possible to "unassign" a previous assigned requisition.