If you no longer need a site in SharePoint Online and you're one of the owners of that site then you can delete it. Keep in mind that once the site is deleted so will the associated Office group and anything linked to that e.g. plans in Planner, a team in Teams etc. (if your site is a SharePoint team site)

Steps to delete your site:

1. Login to your site in SharePoint Online
2. Click the cog in the top right part of the page
3. Select "Site Information"
4. At the bottom of the Site Information section click on "Delete Site"
5. This step cannot be un-done, be sure you are on the correct site before proceeding: Confirm you're sure to delete the site and office group

Note that the deletion can take some time (minutes or even hours if it's large), so your site may continue to appear in SharePoint or search results for up to 24 hours.