Maybe easier is to just ask that they show their hands while you ask a short question until they gave the answer. Could even be up front about it and say you suspect they're looking up the answers, since it's not like you care much if they get upset at a false suspicion, or just say "to avoid looking up answers, our standard procedure involves this".

The drawing approach also sounds like a good idea, though it's not like software is not going to evolve to be able to draw answers graphically which the candidate could copy down. By having them not able to input something into the machine, the only remaining option is someone listening in and feeding the answer on screen. Plausible, but that's a level of being prepared to cheat that the helper could also prepare to draw stuff out. Or they type with their feet but that's also a scenario where I'd be happy to have them come in for a final interview and demonstrate this amazing ability!