Can someone tell me if there is any TN that explains how to unistall operation manager and also remove the Monitoring database completely from SQL 2005.
No technote I am aware of. Operations manager is pretty much just a database and a virtual directory in IIS.
May be try just removing them ensuring you have a backup of SQL database and IIS before so you can put it back if start gettings loasds of errors. Doubt you will though.
Otherwise log a case if you have a support contract.