zlacker

There are a couple of other options, although I'll agree that none of them are ideal:

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.