You can check a friend's online status from Friend List in your user page. Each user can register up to 300 friends.
Select a friend to display more detailed information about that user.
Making someone a best friend will send them to the top of your friend list. You can also select "Options" to delete a user or add them to your blocked-user list.
What happens if I block someone?
You will not receive friend requests sent to you by blocked users, and you will be unlikely to encounter those users online (excluding some games and game modes). You can block users from your friend list or by selecting Add Friend ⇒ Search for Users You've Played With.
Blocked users will not be notified that you have blocked them. You can check your blocked-user list and unblock users from User Settings ⇒ Manage Blocked-User List in your user page.