The question of deleting blogs comes up frequently. What happens when a user deletes their blog? A site admin? When do you a delete a user’s blog?
Well, the answer is- it depends.
When a user chooses to delete their blog, they are asked to confirm. This doesn’t really delete it from the database, rather it is marked as deleted. The site admin will see this highlighted in red on the blogs list. (Site Admin -> Blogs) Visitors to the deleted blog will see a mostly white page and a message about the user electing to delete the blog. The blog still exists in the database, and the site admin can UNdelete the blog in the backend if a mistake has been made.
If you are concerned about database size, the yes, as a site admin, you can permanently delete the blog.
The down side of full deletion is that sometimes sploggers will search for previously existing blogs and reuse the name. It’s really your call. You could add the old blog name to the backend under Site Admin -> Options -> Banned Names ensuring it does not get used again.