Server sends the client a "Prevent the running of gestures" message.
Anything that refers to a gesture from there will get an error, so people won't be able to use them via huds.
Since the parcel is doing it then land permissions and rezzed items can gesture.
Old viewers would circumvent this for a time but the second phase would be a "do you have a gesture disabling script built in?" when the client tries to connect. If not then it fails with an error to that effect.
Alternatively, gestures are also a called script that can be enabled / disabled. So they are identified as an item. Thus the server probably stores them in a seperate database that is called. Much like how your cloths might not be accessible but you are in world and objects rezz around you. So too could gestures be distabled with "Are they trying to access this gesture in a parcel where it is banned? If yes then stop the query and return an error."
That is a completely server side solution to enforce this.