How do i report someone who will not stop contacting me on messenger?

i have some one who will not stop asking for authorizations to be added to my messenger list. i have rejected this person every time because i do not know them and do not recognize their yahoo i.d. i've asked them for clarification but they refuse to provide it. i can provide the i.d. they gave me, but i just want them to stop, or to find a way to block them permanently...