I just came with the same question and what is what I have found:
go to configuration of UddeIM -> Blocking -> Enable blocking system -> Yes, then when you or member open his/her message, there will be link "block user" right under the link "delete".
roberpc wrote:Hello
Please, could you tell me if you know how to block users using uddeIm with CB.
In the Administration I set "Enable blocking system" but I don´t know how to block in the frontend.
Thanks!!