We had the same problem and where able to delete them via this SQL statement.
Make sure you set the DELID (1729) to the Work Item ID that you want to delete.
Tread Lightly.... I am no expert but it seams to work for us.
Declare @DELID int
set @DELID = 1729
DELETE FROM [TfsWorkItemTracking].[dbo].[WorkItemLongTexts]
WHERE ID = @DELID
DELETE FROM [TfsWorkItemTracking].[dbo].[WorkItemsAre]
WHERE ID = @DELID
DELETE FROM [TfsWorkItemTracking].[dbo].[WorkItemsWere]
WHERE ID = @DELID
DELETE FROM [TfsWorkItemTracking].[dbo].[WorkItemsLatest]
WHERE ID = @DELID
Charlie