There is a scenario in which customer should restricted for sales if overdue found for more then 25 days. and also interest should be calculated for this.
How is this possible in erpnext?
Not by default. You can have a server script to freeze/disable, customer. Interest calculation is also similar.
EDIT: ideal first step would be to trigger a notification if the invoice is unpaid.
The requirement is slightly disjointed process but can be achieved:
1. For Blocking Sales:
You will have to write a server script to block any new invoices or sales documents such as order or quotation if the credit period has exceeded.
ERPNext allows you to block new sales if the credit limit (amount) has been breached. You can refer to the code and come up with similar logic for a credit period.
For unpaid invoices, easier server script reference would be to simply check if a customer has invoice unpaid status for more than 25 days.
2. For Interest on Invoice