Option button on form not allowing deselection

[marathonlady]

I have an option button on a form. When I select it it works properly, which means it allows users to select from a list of names below it. When I deselect the button, I get a message that says I am being prevented from saving data in that field.
In the event BeforeUpdate is where I have my code that says
if the button is false, and it's selected, set it to true. If it's
true and updated, set it to false. Help me, please. What is the right code I need.