Information
Based on Microsoft design an account with DB_OWNER can elevate permissions to SYSADMIN
MSDB must be configured with the TRUSTWORTHY flag ON to work properly. If the TRUSTWORTHY setting is set to ON and if the owner of the database is a member of a group that has administrative credentials, such as the sysadmin group (for example the default sa login), the database owner can then be able to create and run unsafe assemblies that can compromise the instance of the SQL Server, as well as run code to elevate his privileges to SYSADMIN
Solution
USE [msdb]
GO
ALTER ROLE [db_owner] DROP MEMBER <username>;