Depending if you are using a hosting or an on-premise server the key delete option is different
A key can be deleted there are no transactions for this key
A key can be deleted independent of existing transactions by an administrator user
A key, which can be deleted, has a delete icon in the Key List and a Delete button in the Key Details page
To delete a single key, select the key in the Key List, click on the key to get the Key Details page. Then click on the Delete Button
The Delete Button is only available if a key can be deleted (condition see above).
Multiple keys can be deleted at once. in the Key List click on the TAB Mass Delete Keys
You will get a list of all keys which can be deleted.
Select the keys you want to delete and click on: Delete selected records
When you delete a key, all attached notes and license files are also deleted