One form of hacking or another, prevention must occur in both types. Old school is busing people you want to vote a certain way to the booths and leaving others behind, having some vote at multiple stations, even through absentee votes for people who you know won't vote or cannot; the dead being an example.

I would say than an effective electronic system would need to be open from code to monitoring. There is no reason to have the voting machines nor machines which accumulate the vote to be connected to any external network.

You have to start trusting somewhere.