We have several ways to flag these and isolate them. If our sales reps determine that the lead is "Mickey Mouse", they change the Lead Status to "Bogus Info" and if the lead is no longer at that company, they change the Lead Status to "Gone". In both cases, this triggers a smart campagin to change their lead revenue stage to "Bogus Info". We also periodically move leads with Email Invalid and that have had no activity in the last 6 months to the "Bogus Info" revenue stage. We haven't done it yet, but they be exported (all columns) and deleted as I've never seen a lead in the Bogus Info revenue stage come back to life.