If you use Tags and view the tags listing page in admin cp that might span 4 or 100 tag pages, e.t.c. There is no option like with some other things to either mass delete all, or tick check-boxes on a tag pages to quickly delete more than one tag at a time. You have delete each tag individually.
If not feasible to have mass delete all because too many tags could be listed, at least have each tag page use check-boxes to delete all quickly (per page), or able to select a few tags using check-boxes to delete them (per page).
If I arrange tags with 0 use to be listed first top of page, I have to delete each one in turn. When there could be check boxes there to delete them all at once per page... if tags with 0 use spanned 4 pages.