Broadcast Message from root@
The command Ingmar Hupp provided removes all access to your 'wall' program. This 'write all' program will send a message to all users logged into the system at one point. 

I am assuming no one else has access to your machine, so perhaps there is a program you use that makes an errant (or perhaps malicious) call to /usr/bin/wall?

By changing the permissions of /usr/bin/wall to 0, no internal or external source should be able to read, write or execute the program.

Hope this explanation helps!

One more thought: Have you updated your OS to the most recent version? I have come across other instances of this where users traced the issue back to a security update released by Apple.