Account Blocking & Muting
Next to the follow button is three small dots. Clicking these dots opens a drop down menu. From here you can mute an account, block & report (as well as other general options.) Blocking a user stops them from seeing what you tweet.
You can view a list of users you have blocked or muted in your Account Settings. When a blocked user clicks your account they will be met with a message telling them they are blocked.