Email them at support@protonmail.zendesk.com
. Hopefully if enough complain, they will add a toggle.
They also have a community cli client where the one example of a nag message is easy enough to remove on line 80 of cli.py https://github.com/jonasjancarik/protonvpn-cli-community/
Eviction resistances happen all over the world. Join your local tenants union.
In the UK, ACORN covers most places, but there are also area-specific unions such as London Renters Union and Greater Manchester Tenants Union. There is also the Advice Service for Squatters who can help support squats.