What happens to unfinished streams on Swarm?

The short answer: Swarm will delete the data automatically.

The long technical answer: As long as we have not seen all the data for a stream, it will not have a UUID, and will be considered trapped capacity. This trapped capacity is not part of Used Capacity, as soon as the volume is low on available space (below a 20% threshold), it will start defragmenting the volume and will reclaim the trapped capacity. (Health processor does this in the background).