Right now, only the PR owner see the "delete" button in the list of PR for a given repo.
The repo admin should also have access to that delete button to delete broken PR (ex: when a PR has accidentally created with wrong source and destination resulting in a gigantic diff that takes a lot of time to render the PR to the browser).
See thread on the mailing list https://lists.sfconservancy.org/pipermail/kallithea-general/2017q3/002396.html
Thomas De Schampheleire has a slightly different idea on how to tackle this: https://lists.sfconservancy.org/pipermail/kallithea-general/2017q3/002399.html
Regarding deletion via the 'My pull requests' page: I find the 'delete' button for the owner too inviting. In fact, I consider the need for deleting a PR a highly exceptional situation, purely reserved for wrong/accidental creation of a PR. For all other uses, PRs that are no longer needed should be closed, rather than deleted.
This topic was also discussed at an earlier occasion. I think one idea was forbidding deleting a PR that has had activity, e.g. comments or status changes, for regular owners. Admins should still be able to do it, though, via the web UI and obviously via direct database access.
To improve the management aspect for admins, avoiding the need for direct db access, perhaps a UI page could be added, allowing to filter PRs on different criteria and with checkboxes and buttons to allow (batch) closing and deletion. Perhaps, the filtering aspect would be useful for regular users too...