Override WndProc in your Control and do the following. You should then listen to keyup and show the context menu yourself.

[C#]

private int const WM_CONTEXTMENU = 0x7b;
private int const WM_KEYUP = 0x101;

protected override void WndProc(ref Message m)
{
  if ( m.Msg == WM_CONTEXTMENU ) return;
  if ( m.Msg == WM_KEYUP )
  {
    Keys keys = (Keys) m.WParam.ToInt32();
    // Prevent this key from being processed.
    if ( keys == Keys.Apps ) return;
  }
  base.WndProc( ref m );
}

[Visual Basic]

Protected Overrides Sub WndProc(ByRef m As Message)
If m.Msg = 0x7b Then 'WM_CONTEXTMENU
Return
End If
If m.Msg = 0x101 Then 'WM_KEYUP
Dim keys As Keys = CType(m.WParam.ToInt32(), Keys)
' Prevent this key from being processed.
If keys = Keys.Apps Then
Return
End If
End If
MyBase.WndProc( m)
End Sub

Contributed from George Shepherd's Windows Forms FAQ