Payment Document Number is open for change/update manually or override..how to restrict it.

Hello,

Dear all i have a requirement where Payment document number is open for change users can easily update the document number manually to override the automatic document number, the document numbering has been setup from manage payment documents, which should generate automatically, but it is allowing to change it manually where as we don't want this option top be open with users to play with the document numbers, so i is there any way this option can be restricted to be able to update by users...

Thanks & Regards