Unfortunately, there's no such option in Bitbucket at the moment. Any user with write access to the repository can decline a pull request.
This use case makes sense to me, and in fact we already have such feature request in our public issue tracker. However, looks like it was deprioritised in 2018 due to low interest. I think it is still worth raising a new one so that it is triaged again – do you mind doing that? You'll be notified on status changes for that ticket.
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