Facebook-click on their profile, then on the 'message' button dropdown and you will see the option to 'unfriend'. you can also block a person this way.
Twitter-to remove or block someone on twitter, click on the button with a head icon on it next to the 'follow' button on a user's profile. If you click on this you will see a menu with the options to BLOCK the user to prevent them from seeing your profile, and vice versa, and you can also REPORT FOR SPAM, which will alert twitter to any users who are abusing the service. Read our article about twitter safety.
YouTube- go to your account page and click on ''all contacts'' link in the friends and contacts'' section. Choose which person you want to unfriend and the click on ''remove contacts''. From then on the person won't be on your ''Share Video'' list.