Hi There,
In answer to your question, Anne-Marie, the way Marketo handles hard bounces would merit removing those leads from your database.
A little further to what Melanie said - Marketo definitely DOES differentiate between Hard and Soft bounces. A hard bounce will be marked as Invalid = true, and will not be marketed to again, unless you change them back to False. A soft bounce will be marked as Suspended = True, and you may notice in the Lead Info that you actually cannot change this to False. That's because when a soft bounce occurs due to a spam complaint from someone such as SpamCop, the email address cannot be marketed to for a certain period - generally 24 hours. After the 24 hour period, you will be able to market to them again, however, it will still show in the Lead Info as suspended for that period.
If a lead is marked as Invalid, there really is no reason to keep them in your database, unless they were marked invalid for some reason other than being marked as spam. To change them back and resend to them in subsequent campaigns may be inviting trouble - and more spam complaints. Suspended emails are still very much marketable after the suspended period is over, so there is no need to delete them.