The Delete Delete Site Plugin removes the "Delete site" link from your WordPress multisite admin panel
After you've downloaded and extracted the files:
Note: This plugin does not remove the ability to delete the site, it just removes the link from the admin panel which should prevent users from inadvertently deleting their site. This plugin intentionally does not work if you are a "WordPress Super Administrator". This ensures that you as the owner of the site have full control over the admin panel, yet anyone at a lower userlevel (ie: "WordPress Administrator") will not see the link.