Keep a playpen handy. I give 3 strikes you are out! I agree, they know what NO means. Also, they know that NO with a big smile can get them a free pass. So, I have stopped using no I use "STOP" and will clap. I do the same thing as you with the catering to hurt baby, etc.
After the 3rd strike put the offender in the playpen right outside the playroom. After a 2 minute or longer break bring them back in. If it continues put the offender away for longer in another room.
I find I always have some kind of hitting or agressive behaviour when a newbie starts.
Good luck!