Jira: deleted user coming from external directory (like LDAP)

You are using Atlassian Jira, but your users are authenticated from LDAP (AD/whatever)?
Surely, when one of your users leaves or gets her name changed, you'll be stuck with malfunctioning watcher lists, as Jira does not use any of those pesky "relational" stuff in RDBMS-s (valid of any DBMS vendor).
Get rid of the faulty associations.

DELETE FROM userassociation 
WHERE LOWER(source_name) IN (
    SELECT DISTINCT LOWER(source_name) FROM userassociation
    MINUS
    SELECT DISTINCT lower_user_name FROM cwd_user)