In short, it’s not easy and perhaps impossible if someone is really determined. If you display a 3d model on the internet then people can download it, in much the same way as if you show a video on the internet people can record it.
Probably the best and simplest approach is just to make sure that the license is clearly stated somewhere. Some people will ignore that, but you’ve got to weigh up the costs of putting huge amounts of effort into preventing this vs the costs you lose from unwanted downloads
What kind of app are you making where you think this will be a problem?