You may only delete streams with zero items sold. To do so, within the Online Shop Portal, select the stream you’d like to delete, and click "Edit Stream" on the top.
If this is a future stream, the window that appears will allow you to edit a stream's title, as well as the scheduled start time for that stream in the instance rescheduling is necessary. This is specifically useful in rescheduling previously scheduled Streams.
If you wish to delete the stream, simply click "Delete Stream & Items", and that Stream will be removed. Streams with sold items cannot be deleted, as those sales must be fulfilled or refunded.