The first thing I would do would be to report this clown to the admins. Unfortunately there's no easy way for you to block him and only him.
The command to keep other users from writing messages on your terminal is
mesg n. The
mesg y command then allows writing again.
What this command does is to change the permissions of your terminal device file (e.g.
/dev/pts/8, etc.) to allow or disallow other users from writing to it. Typing
who am i at the command prompt will often show you the name of your device, with the
/dev/ prefix stripped off.
The traditional UNIX permission system allows you to set permissions separately for yourself, for a single user group, and for all other users. If you and everyone you want to talk to happen to be a member of a group that the h4x0r is not in, it's possible you could
chgrp mygroup /dev/myterminal to set the file's group, then
chmod 620 /dev/myterminal to set the permissions. (
620 is shorthand for "user can read/write, group can write, all others have no permissions".)
Applying an access control list to your device file might be a way to do what you want, but that's system-dependent and non-standard, I've never done it, and it could cause problems for others if the login system isn't expecting it.
Finally, for more info (just in case you don't know already), try the
man command --
man chgrp, etc.