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 the label is removed from all issues, it will no longer be seen when browsing a labels. However, the deleted label might still show up on the browser immediately after deleting the label from all issues. Please be aware, it might be caused by index/cache replication eventually will become consistent or browser cache would need to be flushed and reloaded.
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?
In Jira, you can search for a label and perform a bulk edit operation (see Edit multiple issues at the same time - Jira Cloud or Edit multiple issues at the same time - Jira Server and Data Center) to remove it.
Both Cloud and Server application users can also watch the following feature request for improved label management in JIRA applications: