How to delete tags from a large number of files
You can delete custom tags you created by going to "Finder-->Preferences-->Tags"  Then highlight the tags you don't want.  (To select multiple at once click one and then shift+click the last one).  Right-click and "Delete # Tags".



In your case, with 70+ tags, just use select all from the edit menu (Command A) to select all the tags. Then hold the option key to click the 6 tags you want to keep. Once the rest are selected you can mass delete them, leaving the few tags you prefer.