• Select Options > More Options... from the Windows Live Hotmail toolbar.
• You will see Junk e-mail, click the Safe and blocked senders link under Junk e-mail.
• Now click Blocked senders link.
• Type the undesired email address under Blocked e-mail address or domain:------@----- and click Add to list button

You can block all addresses at a domain — the domain being what comes after the '@' sign in an email address — by entering just that