Please, read my comment to the question first. I've mentioned there about good programming practice. So, your code can be changed to:
Private Sub MarkCrewActiveToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles MarkCrewActiveToolStripMenuItem.Click, MarkCrewInactiveToolStripMenuItem.Click
Dim tsmi As ToolStripMenuItem = DirectCast(sender, ToolStripMenuItem)
If tsmi IsNot Nothing Then
Dim myarg As Integer = If(tsmi.Name = "MarkCrewInactiveToolStripMenuItem", -1, 0)
UnCheckCheckedListBox(myarg)
End If
End Sub
Private Sub UnCheckCheckedListBox(Optional ByVal action As Integer = 0)
If TypeOf Me.ActiveControl Is CheckedListBox Then
Dim objControl As CheckedListBox = Me.ActiveControl
objControl.BackColor = If(action=0, SystemColors.ActiveBorder, SystemColors.Window)
objControl.Enabled = action
Me.Focus
End If
End Sub
As to your main question...
Seems, you have to move focus on the other control or
disable selection[
^]...