Time Machine Backup - Best Practices

Am responsible managing MacBook inventory for small team. All users are also given an external disk to perform regular Time Machine backups. Looking for advice/best practices when it comes to reusing/repurposing the external disks when an employee has left the company. Currently, when an employee leaves, external disk is unable to be wiped/repurposed as its been encrypted by the former user. Is there a way around this? (or would this be a function of the disk, something the manufacturer would need to answer?)

Thanks in advance.