How to deactivate deletion of Invoiced Sales order? Please suggest.
On the Updates tab of the AR parameters set the "Safety Level of Invoiced Orders" to "Locked". That gets you two warnings on deletion. However to really prevent it you need a security role to prevent the deletion on the table, which presents an issue when you incorrectly create an order and want to delete it.
You should have no required fields in there and AX has all the data it needs in the posted transactions, so being able to delete it can be required.