Found under Mass Mailer → Never-email list. Anything on it is skipped by every blast, including ones already queued.
Adding entries
Paste addresses or upload a file (up to 1 MB). You can block a single
address, or a whole domain by entering @acme.com or
acme.com. Up to 50,000 entries can be added at once.
What else shows up there
The page also lists contacts who unsubscribed and contacts the Email Validator judged undeliverable, so you can see everyone who will not be mailed and why.