Ok so you dont want sent items saved. There isnt a way to do that using Outlook Web App, just outlook as you have seen.
https://support.microsoft.com/en-us/office/change-where-sent-email-messages-are-saved-bd95ef3b-8c04-466a-8576-d1ce0eabeb2c
The other option is to use an retention policy that removes the sent items and apply to that mailbox:
https://learn.microsoft.com/en-us/microsoft-365/compliance/set-up-an-archive-and-deletion-policy-for-mailboxes?view=o365-worldwide
Otherwise you will have to remove manually.