Answer
As long as the account owns no schemas, database objects, scheduled jobs, triggers, or similar artifacts, the database-level account can simply be dropped directly (DROP USER <username>) while leaving the corresponding platform user record inactive rather than deleted — this is safe specifically because an inactive account with no owned objects has nothing left depending on its database-level existence. A combined "delete everything together" option also exists if removing both the platform record and the database account at once is actually what's wanted, but that's a different, more permanent action than keeping the platform record for historical reference.