It is not possible according to GitHub support
I asked support support@github.com on February 2018 if it was possible to remove the Pull Requests tab, and the reply from @nickcannariato was:
Thanks for writing in! We’ve definitely heard some users requesting this feature over the last couple of years, and it’s currently on our feature request list. I can’t make any promises about if or when we may add this ability in the future, but I’ll add your +1 to that feature request so the team can see it.
confirming that it is not possible.
Request on the de-facto trackers:
- https://github.com/isaacs/github/issues/1191
- https://github.com/dear-github/dear-github/issues/84