Email services that comply with laws must post a link in each email that allows recipients to unsubscribe, and clicking this link is usually the easiest way to unsubscribe. Email clients also let users block emails or unsubscribe from lists.
The CAN-SPAM law sets a number of regulations for individuals and organizations that send bulk emails. The unsubscribe link in an email might be small, but it's typically located at the bottom of the message. Google's Gmail has an unsubscribe button displayed in the header of the email that users can click if they have trouble finding the unsubscribe link, and email clients allow customers to block emails from certain addresses if they choose.