how do i remove offer@newsletter.sale

if this is Apple iCloud and the source address is consistent, set up a filtering rule on iCloud.

Set up rules to filter email in Mail on iCloud.com - Apple Support

If the filtering rules do not apply, then you’re getting spam, and spammers usually welcome your replies and feedback. Replying to spam attracts more spam, as you’ve provided proof that your address is valid, and that you’re willing to interact with a spammer, and such email addresses are worth more money when resold.

Keep marking them as junk and eventually they will be sent to your junk folder.

No, that's your email provider allowing it. Maybe talk to them.