The only way to prevent this with ISP gateways is...

projects like http://www.neufbox4.org which aims at creating an alternative and entirely open firmware for the gateway

ISPs usually break the GPL by using free software and not redistributing, and their gateways rely on security by obscurity.

The customer is then dependent on the firmware upgrade from the ISP following the discovery of a vulnerability, and some times it can take ages before it is corrected.

When the community is in charge of an alternative firmware, vulnerabilities are spotted earlier and corrected faster.