Moving your thread to the Photoshop Elements Forum.
I don't have a Mac, but there should be an equivalent to the contextual menu opened in Windows by right clicking. That contextual menu when you select a tag offers the choice to delete the tag.
If you want to delete tag entirely, right click on the respective tag in the Keyword Tags panel and choose "Delete"
If you want to remove a tag from a file, select the file >> right click >> Remove keword tag