Open the Messages app
Tap Edit or Filters
Tap Recently Deleted
Select the messages or conversations you want to remove permanently
Tap Delete
Tap Delete Messages to confirm
To clear deleted messages from iCloud, go to Settings
Tap your name
Tap iCloud
Tap Messages
Turn off Messages in iCloud if needed
Delete any synced messages from other Apple devices if they still appear
Empty the Recently Deleted folder in Mail if you mean deleted email messages
Restart the iPhone if deleted messages still seem to appear
Update iOS if the deleted messages do not clear properly