You can not disable PRs on github. The wiki yes, issues yes, but it's not possible to disable PRs, that simply isn't an option.

At best you can add a note/contribution guideline saying you don't accept PRs, and close them as soon as they're created, but it's not possible to prevent them.

Which would remove the most obvious place to look for commits for this new fork, or for this patch that you were just about to write.