To quickly and easily delete multiple contacts, you can import them as a Delete List. Go To Audience > Manage Contacts > Import Contacts. Click Remove Contacts > Delete Contacts. Pick the via Cut & Paste or via CSV option depending on which you prefer:
This will mark any contact on the list as deleted. They will be moved to your Non-Subscribed folder with a status of Deleted.
If you re-import these contacts, they will become active again as long as they have not unsubscribed themselves from your newsletter.
You can also manually select contacts in any list and delete them: