Disabling modded clients is impossible.
Having a separate modded client to join MCGamer, however, is possible.
There are several issues with this, including:
- More development/money required on MCG's end
- Players may not understand how to install the client
- Inconveniences involving switching servers may irritate players
- Hacked clients for that modded client could be made as well
There's no way to get rid of hacking, and that's what moderators are for. The server network will remain as it is.
With a normal plugin as an AntiCheat, detecting hacked clients automatically is impossible. The client must send a packet to the server in order for it to be identified, otherwise it remains hidden. This is what allows BSM to be blocked, as the creators of the mod have implemented a packet delivery service within the mod so servers can ban it as they please. (There are bypasses for this as well, even on MCSG... I won't say anything else.)
The closest thing there is to a perfect AntiCheat is anticheat, which is still very flawed. The creators of Minecraft, namely Notch, are perfectly fine with hacking and modding in Minecraft. Mojang was decent friends with the members of Team Avolition, a notorious hacking/griefing team.
I think MCGamer is doing a great job at banning hackers, though. They're doing all they can.
Just give it time.