It's a pity, but as a rule you cannot do anything with it since this occurs if:

• You try to send a message to a user under the age of 14 without being his/her friend or a friend of his/her parents
• This user has blacklisted you Sorry, but you cannot get off the Black list on your own or with the help of the website's Administration. Only the owner of the Black list can decide whether to remove you from it
• A user has disabled receiving messages from users who are not his/her friends

At least now you know what has happened and can arrange things with the specific user outside the website to make sure that everything is OK!