I think that Mathew is saying that a script could prevent the save and I'm saying that natively I cannot prevent the save but I can flag the record. I don't know your business process, but sometimes a solution is that the user first enters the customer and saves. Then the edit the record and the rest of the data entry fields appear along with the warning message, if any. If the warning message appears, then the user will know and also a form rule could hide the rest of the data entry sections.
That way the user will not waste time with a lot of data entry needlessly and can "cut their losses' more quickly.
