When you block someone they will no longer be able to do things such as tag you or see things you post on your timeline.

To block someone:

Click in the top right of Facebook.
Select Settings & Privacy, then click Settings.
Click Blocking in the left side menu.
In the Block users section, enter the name of the person you want to block and click Block.
Select the specific person you want to block from the list that appears and click Block > Block [name].

Note: If you can't find someone using this method, try going to the person's profile and selecting Block from the menu in the bottom right of their cover photo.

Keep in mind:

When you unblock someone, you won't automatically be friends again. If you block a friend and then unblock them, you'll need to send them a new friend request.
People won't be notified when you block them.