We've fixed the problem with CPU spikes in NPM 10.6 RC so I would recommend to try this updated.
rgward has mentioned that here: http://thwack.solarwinds.com/ideas/2610 If you would like to complete disable multicast (not per node via List Resources), you can do that via SQL query: DELETE FROM Pollers WHERE PollerType LIKE '%multicast%'
I believe that you guys would like to control multicast (and other polling) globally, please vote here: http://thwack.solarwinds.com/ideas/2610