Originally Posted by fysiks
Changing the rcon password is good advice. If a password is required, use a very strong password that cannot be guessed.
The "say_team @" message will begin with either "(ADMIN)" or "(PLAYER)". "(ALL)" is only shown when using amx_say. If it's a player doing this directly from their own game, it will show up in the logs so that the person can be banned. If it's going through RCON, the change of password will do wonders.
You're right about that, apologies for the mistake.
In that case, you need to also remove that particular player from the users.ini file, because they probably used amx_addadmin to give themselves flags using rcon. A plugin wouldn't help in your case.