Why shouldn't farming be forbidden?
Because not every farm is a cheater, it's as easy as that.
A lot of guys, like e.g. labude, achieved everything they wanted in this game. So after that they decide to buy a bunch of young guys and train them as they want. Is that cheating? It might result in farming, if his players are good enough to get into any NT.
If a guy just doesn't want to compete but train a bunch of his own draftees for himself because he wants to have a team consisting only of his players - is that cheating? E.g. Scott Broszeit got trained in Div.4 till he became a NT player for Germany. His owner had to sell him some time ago because he couldn't afford him any more. Does that make him a farm or not? A lot of NT players have their roots in Div. 4...
If a player doesn't want to compete anymore but has a lot of money and wants to help any NT by training and farming an NT-player? Why do you want to forbid it?
As you say: if a farm is a multiaccount it's already forbidden, because that's cheating. If you hate cheating: help the GMs catch cheaters.