How do you stop someone sending emails to you?

How do you stop someone sending emails to you?

I'm always getting emails from someone that I don't wish to talk to. I figured out how to send all their emails to my spam automatically, but even then I have to clear my spam and trash to get of their mail, and I'd much rather find a way of preventing their emails from coming in all together. Does anyone know of a function to prevent these sorts of emails?