Hackers running around on my servers?

[fredmg77]

Game: TF2
Admin Mods: Sourcemod, Metamod

My server has VAC enabled and is a dedicated server, I have run it for a year or so about now and have had no issues, but lately with a few cross players that were banned, i've had an incident where the chat was spammed.

It said things like:

Console:WWWWWWWWWWWWW

How would they have done this? Is there a security issue? Or was it just simple hacks?
I'm quite disturbed it said Console:

[rogerxiii]

They could have your RCON password, which would be VERY dangerous as it could damage your whole server. I would recommend updating the password and installing SMAC, SourceMod Anti-Cheat, the best anti-cheat plugin there is.
https://forums.alliedmods.net/forumdisplay.php?f=133

EDIT: I just realized you might not have even chosen a password. Add this line into your server.cfg:
rcon_password "PASSWORDHERE"