Jonathan Almquist has posted (a while ago) an article on how to clear discovered objects after you have disabled the discovery rules in OpsMgr that I think deserves a notion.
Read more about it at Jonathan Almquist on Operations Manager : Remove-DisabledMonitoringObject.
About Sam T
I am a System Management consultant focusing mainly on System Center Operations Manager, System Center Opalis some Microsoft SQL Server and OP5.
Besides doing consulting I am also an MCT and are holding both the official System Center Operations Manager courses at all levels (50028, 50216, 50231) at Cornerstone and Global Knowledge and holds customized classes at customer sites.