Wall is a great command if you’re intent on getting notoriety/kicked off the box you’re on. It broadcasts a message to every terminal open for every logged on user. For this reason, it may be turned off on the computer you’re on. This means every xterm, gnome-terminal, etc and every one of the 6 VTs will get the message. Usage is:

wall “message to everyone”

If you really want to be annoying, type:

watch wall “message to everyone”

this will repeat the message every 2 seconds! Please only do this on your own box or for a very limited time or everyone will want to kill you. Have fun!