It's easy! But it makes us sad that you've had to do that.
If the person you want to block currently follows you, then you simply have to go to their profile page and click the options icon on the top right of the screen. From there you'll be able to block them.
Just be warned that blocking someone means they cannot follow you or see your content. Furthermore, any content you've ever received from them will be automatically deleted. So do think long and hard before blocking someone!