At its simplest:
Sub Demo()
Application.ScreenUpdating = False
Dim i As Long
With ActiveDocument
For i = .Bookmarks.Count To 1 Step -1
With .Bookmarks(i)
If Left(.Name, 1) = "_" Then .Delete
End With
Next
End With
Application.ScreenUpdating = True
End Sub
Alternatively:
Sub Demo()
Application.ScreenUpdating = False
Dim i As Long
With ActiveDocument
For i = .Bookmarks.Count To 1 Step -1
With .Bookmarks(i)
Select Case Left(.Name, 4)
Case "_TOC", "_HIK": .Delete
End Select
End With
Next
End With
Application.ScreenUpdating = True
End Sub