Even if you try to stop mob pking and restrict PK to towers only you will end up with towers being clustered and over crowded which also ruins fort PK. When mob PK wasn't even a regular occurrence the hallways at mod fort and the flag room was rather over crowded at certain times of the day and moving around was a pain the arse and with the screen freezing as well which wasn't fun at all, which to be honest killed pking for me, I saw where PKing was going so why bother?
Even if there were PK zones actually designed for being a pking area people will use it to mob PK with their buddies and then advertise it in their nickname so basically whatever you do there will always be some sort of mob PK. The game has got too crowded now though it's not like it used to be and I don't think you can do anything.
The people who make money of this game don't care at all it's a shame really Graal to me has been one of the most sociable, simple and easy to play games. It has so much potential but they don't care all, all they want is your money.