How to delete a label in Jira
To completely delete a label from a JIRA instance, the label must be first removed from all the issues in that instance. Once it is removed from all issues, it will no longer be seen when browsing a labels.
If the label is still showing as a suggestion after you seemingly removed from all issues it means you might not have visibility to all issues with it.
How do I bulk-remove a label?
If you are using JIRA Cloud applications, you can search for a label and perform a bulk edit operation (see Editing multiple issues) to remove it.