Is there a way to make Github un-approve a pull request if a new commit is pushed to the branch?

On GitHub, navigate to the main page of the repository.
Under your repository name, click Settings.
In the left menu, click Branches.
Select the branch you want to restrict using the drop-down menu.

Select Dismiss stale pull request approvals when new commits are pushed.

This dismisses a pull request approval review when a code-modifying commit is pushed to the branch.

Click Save changes.

Ref: https://help.github.com/articles/enabling-required-reviews-for-pull-requests/