You only prevent Custom Firmware. If you care, In the first place you need to stop using miners and giving service to them. Even firmware provided by the miner manufacturer
Kano must have some weird explanation for why it's okay to allow stock firmware that violate the same license, he did mention it before, I just can't remember it, it probably didn't make any sense to me that's why I forgot, I would love to read the same excuse again.
The explanation is not weird: If one is running a pool then one must allow the OEM firmware because that is what the majority of miners use. It is tilting at windmills to do otherwise.
Now regarding,
You can't prevent them from using your pool.
"Them" of course is referring to miners using 3rd party firmware and yes he DOES prevent its usage. Part of the data exchanged between a miner and pool identifies what the firmware is and his pool software automatically disconnects any miner that is not running OEM firmware. The only exception is BoS because the sourcecode is published and that only works on the Solo pool until it is proven that BoS finds blocks.