Graal always downloads the game files. Else you wouldn´t see them. So no you can´t disable it. Also you can´t force the iDevice not to open something in iFile. Also not giving jailbroken people access to the iServers would lower the playercount dramaticly and with that, also the money GraalOnline earns. Actually it´s profit for them if people steal heads, since players are actually uploading their stuff (and buy Gralat Packs for that some times). Also there is always a way, to get through the "barrier" of Graal files on any jailbroken device.

Heads are always personal, until they get stolen. Same for shields (can also get stolen). There is never a real way to proofe that you actually made the graphic.