How can I set permission for portal user?

Apparently, modifying the Customer role has no significant impact on any portal access.
The default permissions for Customer does not include permission to create or modify any document by a Customer.

But,
I have set up a customer user and with it, I can modify any tasks created by System users, modify any Issue details created/modified by System user.


How can I modify permissions so that Edit Response is not allowed for Customer user?

Please check it.