Auto-remove unused tags
There are a couple of modules that might help:


Taxonomy Orphanage (specifically the cron interface)


  This module provides interfaces (drush, cron, admin form) for removing orphaned taxonomy term references from entities.

Field reference delete (sandbox only)


  This module removes references to a deleted entity from fields stored in an SQL database. It exists to prevent stale references to non-existent content from causing unexpected problems (for example, when the referencing content is being displayed).