Forms :: Combobox With Restriction For Inputting Text

I have a question about for combobox. I have a field from my tables that a text only then I make in the look up property field to become combobox then row "source type is a value list" then,"Allow value list edits, I make it Yes". then in the form its possible for me now to add value list in the combobox. but I want that what ever I input to become a value list, that's all they can select for the value of that field. if they can type or add not listed in the value list I input, it will not possible & there's a msgbox will appear mentioning the "the value you input is not in the list..