Hi Michael

Another way of bulk deleting accounts that doesn't require HR Import is to select users with a filter based on last access date and delete the selected accounts using Bulk User actions 

Alternatively an audience with a rule such as a course completion more than X days ago can be created. The audience id can then be used in the user filter above so and the bulk user action performed. 

You can also use bulk action action to send a message to the affected users warning them you are going to delete their account in case they don't want you to do this as deleting accounts is irreversible.

regards