Post nasty comments in your bio, sending unpleasant messages are some of the reasons why you are in the need to block some of your Facebook contacts.
From this point, the blocked contact may not see what you post in your profile or start a conversation with you or tag you in photos.
How to block a contact on Facebook
Sign in with Facebook and click on the small triangle in the upper right corner. Select Settings > Blocking.
Under Block users, enter the name or email of the user that you want to block, and then click Block. You can also locate the contact in the displayed list and click the Block button.