You can't filter deals and see just those that have no activity in the last 30 days - other than by changing the filter manually each time.
The closest thing I could see would be to view deals where the last activity was 30 days ago - if deal activity is typically regular, this would turn up the ones that have been missed.
Seems like it would be usful to have an 'is not' filter as well as an 'is' filter in the deals view. Perhaps that's one for the ideas forum.
Hope this helps.