Hello,
In OneDrive you cannot delete files that someone else owns. The right fix is to remove the items from your Shared view so they disappear from your OneDrive everywhere.
Sign in to OneDrive on the web. Open Shared on the left, then Shared with you. Select the files or folders from the unknown person, then choose Remove from shared list. This hides them and stops them from showing up in File Explorer if they were syncing.
If you still see a shared folder under My files or in File Explorer, it is a shortcut you previously added. In OneDrive on the web go to My files, select that shortcut folder, then choose Remove shortcut from My files. If you prefer doing it from File Explorer, right click the folder under your OneDrive, choose View online, then use Remove shortcut there.
If the content is inappropriate or illegal, open one of the files in OneDrive on the web and use Report abuse in the menu. That alerts Microsoft and can prevent further shares from that account.
Try this and tell me what you see after removing the items. If anything remains visible, describe exactly where it still appears and I will guide you to clear it.