Prevent specific users from committing
It's possible to use PullApprove to prevent certain users from being committers on GitHub pull requests. You can use this to achieve a separation of roles (ex. "mergers" aren't allowed to commit).
To do this, in the GitHub branch protection settings you can require pull requests, and require PullApprove as a status check:
Then in your review phases, you can write a requirement to prevent specific people from being committers:
Prevent a specific user from committing
"merger-username" not in pull.committers
Prevent an entire GitHub team from committing
len(set(pull.committers).intersection(pull.organization.teams["team-name"].members)) == 0