You can (from most providers) block emails from a sender or domain as easily as flagging it as spam; however, flagging it as spam has consequences for the company that relays the messages, not the one that sends them.

You're punishing the phone company because a company you had a business relationship with can't magically read your mind that you don't want further calls, after you agreed to a couple sales calls in exchange for a product demo.

That makes you an asshole.