I know most everyone seems to be excited about this for the moment, but let me play devils advocate.
Don't you think doing this will inflate the allie market? People in this thread have already "if you want to keep an allie overprice them". So obviously all good allies will become over priced, hurting low to mid level players.
From an allie perspective, having a good owner matters, especially if you're new. When I first started I had a great owner that gave me game tips, helped me with my build, and even visited the clan I was in sometimes to help with unlocks or slow ebs. If someone hired me he hired me back, if the person that hired me was waring he waited until they were finished (courtesy) and then hired me back. Eventually some allie traders got a hold of me and volleyed me completely out of his reach and I ended up with a jerk that liked to farm his allies. The point is allies are more than a commodity, they are other players and if they want to stay with a good owner then both shouldn't get grief for a banner asking not to hire said allie.
I don't get why people in OSW are getting such a huge break but EE warriors who say "don't hire allies while WOC is cast" do not. OSW's last days and weeks or longer, a wave of conflict spell lasts a day. People in OSW's expect to lose some allies anyway, EE is all timed and no one can go allie shopping in the middle of EE. Seems extremely biased.
There is a measure of courtesy that is nice when buying allies. I always look @ at a potential hire's owner to see if they are at war before I hire an allie, and if an allie has on their own banner that they don't want to be hired then I leave them alone. Conversly, if I am at war, I do put something in my banner often about not hiring my allies, and generally people are courteous enough not to. What you are doing here is killing that notion of courtesy by saying all those banners are off limits and it's a free market, and people shouldn't have to think twice about who they hire. I prefer the mutual courtesy of checking and asking and refraining from buying certain allies in certain situations.
Lastly, if your issue is people hoarding all the good allies and taxing for hire, then go after those people, don't make a blanket threat about banners. If someone wants to keep a bucket full of dead accounts what's the harm? There are heaps of inactives. If someone wants to keep all their alts, or whatever, so what? Those are apparently not the real issues. You can address the issues of allie hoarding directly without making threats to the entire kaw population and going after people who are not affecting anyone else with how they buy and/or keep allies.