Data entry check before updating form in access

I have a data entry form in access DB that requires users to enter a client name and a bunch of other information. After the info. is entered there is an update button that updates the main table linked to the form. I have a table that has a list of clients who should not be entered into the table. I want to create a scenario when the client from the list are entered, there is a warning message that prevents the user from updating the table.

I am open to all ideas that might help me accomplish this. Thank You.