how to stop my niece going on Facebook?

[Greg]

My 12 year old nice has been sending very dirty messages to men through Facebook My brother and his wife are running out of ways to stop her as some of the messages are very unpleasant to be sending to men when they've closed her account but she jus creates a new one they've blocked it on her home pc and on her phone but she goes to friends and does a new account using a false name they've tried talking to her but hasn't worked! Shes not bullying anyone for the record! if anyone can suggest anything be great!!!!! Please no stupid remarks