Hi Kenji,
are u a jira-administrator? I am not sure what u mean with pull request.
But the option to delete another users comment depends on the used permission-scheme in your project.
There is a option for "Delete all comments" permission and who is able to do so.
Best regards.
Addition: Oh I just saw, pull requests are from bitbucket. I am sorry, i am not familiar with it. If this is right, I would suggest to add another tag for bitbucket in order to get more attention on this.
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