Replying to Deleting iPhone texts permanently?

Ok so this may be a dumb question but I need to delete a large amount of texts from a single conversation on my iPhone. It's the conversation with my ex who I have like 150,000 texts from and its taking up a lot of space. If I just click "Edit" and then "Delete" it doesn't get rid of it permanently. It will be impossible for me to go into the conversation and delete each text individually so what can I do? And I even click delete all within the conversation and it just comes back....