In Exchange Power Shell, run one of the following:
The Variable Method:
$move = get-MoveRequest | Where {$_.Status –eq ‘completed’}
Foreach($m in $move){Remove-MoveRequest $m –Confirm:$false}
Or
The One Liner:
get-MoveRequest | Where {$_.Status –eq ‘completed’} | Remove-moverequest -confirm:$false
Ex-Shell 