Originally Posted by KrazyKat
If you think rcon is the problem, remove the rcon_password. rcon_password "" will prevent it from being used by malicious actors. I think this is more related towards the ALL chat spam (triggered by using say_team @), which you can block using OciXCrom's Chat Manager, or a custom plugin if someone's interested in making one.
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.