Deleting individual Submissions is on the roadmap, but we're working on other functionality as part of v1.5. For more details about what's planned for v1.5, see the release criteria. We don't currently have a timeline for when we will add individual Submission deletion.
Now that v1.4 is released, it is possible to purge a Form. When a Form is purged, its Submissions are also purged. At the moment, this is the only time when Submissions are deleted.