- use zoning laws to specify that no religious building shall be built within x miles of the WTC site, grandfathering in the existing churches. (It wouldn't be legal to specify no mosques.)
- use eminent doman to buy all the property within x miles of the WTC site, so the land become government owned, just like the Flight 93 memorial in Shanksville. Of course, the cost would be high. If the public is willing to pay the price, and not spend the money on other uses, then they can have a buffer zone.
Finally, I like Dan Drezner's comments.]