You will only be able to block a member from sending you a Private Message. To do that you need to do the following:

Go to ‘Chat’ > Click ‘Create Chat Room’ > Search for the member you want to block > Select the member, it will open a Chat Room > Click on ‘Info (i)’ icon. > Click on ‘Block member’

Note: Chat Room will not be created for a Private Message unless you input and send a message.