Is there a value set into one of the other fields in the record that then mark this record's "deal" as "Closed"?
If so, a validation rule can reject all changes to the date field when such a value is set to that other field.
There is another field that marks the record as "Closed." However, this same field changes after a certain time, to "Delivered" or "Invoiced." How can I set up this validation rule that you are talking about?
What value in this field shows that it is not "closed"?