Is there a way to block your ISP from controlling the password of your main router?

Blocking ISP access to their own device is not a clever idea.

Instead just buy another router and connect between ISP's router and your computers. Then move all configuration related to your LAN to your router and configure ISP's router as bridge (or put your own router in DMZ).

This way ISP will have access only to the bridge, and not to your LAN.

It's not recommended, but it's possible. Look for an option labeled TR-069 in your router, then disable it. Simple as that. TR-069 basically enables your ISP to change every* setting in your router, and disabling it disables ISP control over your router.

But do take note that if your ISP has some configuration changes (very unlikely), It won't get to your router. In that case, when it fails to connect, turn it on again until internet connectivity is restored. Or do a factory reset.