It would be handy to have 3 options:
Ban email address (less effective for general spam as it is often fake (but helps when some idiot of a company decides to submit a "seo service offer" to your support department))
Ban domain (like above, is less effective for general spam, but stops you getting any email from a certain domain (SEO service offers etc)
Ban IP (Handy if you get a lot of spam from a particular IP or range)