The only way to restrict editing the Due date on a transaction on the ledger is to take away the edit function completely for the transaction.
This can be done by the following:
Head to the System Tab
Click on the Profiles button
Find the relevant profile the user is in and click Edit
Click on Sales Ledger or Purchase Ledger option
Then Records
Then either Customer or Suppliers
Change Edit Sales Transactions and Edit Aged Debtor transactions / Edit Purchase Transactions and Edit Aged Creditor transactions to View Access rather than Full Access.
Click Save