If you have some agents that you definitely removed from your management group by MOM admin console, manually or through the ResKit tool "Cleanup MOM" AND they still re-appear in the MOM console, then try this script:

http://www.huntland.co.uk/Downloads/MOM/RemovingUnwantedMachineEntries.html

(remember to backup your DB of course)

If the agent that always re-appears is a domain controller then remember to delete that agent also from this AD container: CN=MOMLatencyMonitors,DC=ForestDnsZones,DC=ad,DC=pkobp,DC=pl

It's a forest-wide application partition, to delete the computer from that container you can use LDP.exe/adsiedit.msc or any AD editing tool. BTW a great set of free AD tools you will find here:

http://www.joeware.net/win/free/index.htm