To avoid the “The Following queues are in use by default owner setting, workflow rule, sharing rule, email routing address or actual record owner:” exception in Salesforce, do the followingChange the ownership of all the records of the object to be removed from the Queue to an user or to a different queue.
1. Delete all the records for the object to be removed owned by the queue permanently(delete them from the Recycle Bin).
2. Remove the queue references from Workflow Rules.
3. Remove the queue references from the object to be removed Sharing Rules.
4. Remove the queue references from the Assignment Rules if you are removing Lead or Case object.