The fastest way would likely be to add a new field `closed: true` to the bucket, triggering the OpObserver and clearing the bucket from the catalog. This would require a schema change for the buckets collection. It would result in an actual write to the document (which must be replicated), additional space usage, and encodes side effects on the BucketCatalog on this "no-op" op.

A more sophisticated way, which would not require any document alteration, replication, or encoded side effects, would be to introduce a new administrative command to clear a bucket (or a namespace if that's desired) directly. This would obviously require a bit more a heavy-lifting, ACL controls, scoping, etc.

Conveniently, either way, the necessary BucketCatalog functionality is already supplied by SERVER-55060.