Unless the stream is a task stream, deleting the stream only deletes the stream specification, not the files that have been submitted to the stream.
The files submitted to the stream are part of your permanent history; you want to keep them!
I often find myself referring back to the changelists and diffs of changes made many years ago, sometimes even decades ago, so I do everything possible never to erase old history.
If you really wish to destroy the permanent history of changes made to this stream, you can use the 'p4 obliterate' command, but be aware that this command cannot be undone.
If you are contemplating using obliterate to destroy the files submitted to this stream, you should consider contacting Perforce Technical Support first, as the obliterate command is complicated and has a number of options, and you want to make sure you use the right options. And take a checkpoint first, just for extra protection.
If you are using streams for temporary work, and frequently find yourself wishing to obliterate that work, consider using task streams.