In the form Design View, select the combobox and goto Properties - Event and
click in the box for OnKeyDown. When the ellipsis shows up, click on it,
click on Build Code then use this code:

Private Sub YourComboBox_KeyDown(KeyCode As Integer, Shift As Integer)
 KeyCode = 0
End Sub

Now nothing will happen when the user presses any key.

BTW, setting  "Limit to List" to  YES doesn't prevent typing in the combobox,
it merely pops up an error message when the typed data doesn't match an item
in the box.