2) You can use "interaction limits" (in repository settings, under "moderation options") to limit repository interactions to collaborators. This can only be set for 6 months at a time, though, so you'll need to reactivate it periodically.
3) You can archive the project and unarchive it temporarily when making changes, disabling all activity on the fork.
1) You can set up Github Actions to automatically close pull requests: https://github.com/marketplace/actions/close-pull-request
2) You can use "interaction limits" (in repository settings, under "moderation options") to limit repository interactions to collaborators. This can only be set for 6 months at a time, though, so you'll need to reactivate it periodically.
3) You can archive the project and unarchive it temporarily when making changes, disabling all activity on the fork.