You can delete existing ones by putting display order as Thumbnail, then selecting each smart-tag in turn. Once you have all the files with the selected smart-tag,select all the images and just right-click to Remove keyword tag and chose the keyword tag which you use to make the selection. They should all disappear. Go back to Show All and select the following smart tag.............. until you have got rid of all the ones you don't want.
Are you able to delete the smart tags now?
If you dont need the smart tags, change your preferences by clicking on Edit->Preferences->Media-Analysis.
Also, for removing the tag, right click on the images and select the Remove Keyword tag option.