To remove a site collection (classic SharePoint site collection) or a site (modern SharePoint), follow these steps:

1. Navigate to the SharePoint Online admin center (https://admin.microsoft.com | Admin Centers | SharePoint).
2. Click Active sites.
3. Select the site to remove and click Delete on the menu bar. If the Delete button is not visible, click the ellipsis (...) icon to expand the menu bar and click Delete from there.
4. Click Delete to confirm.