It's quite easy to make them disappear or bulk fill them. Like bank notes, paper ballots can be counterfeited.

Canton of Geneva have open sourced their voting app. https://github.com/republique-et-canton-de-geneve/chvote-1-0

Of course there will never be a perfect way. However, I have a feeling that by multiplying voting methods, fraud impact can be contained. Mail vote increases the number of people voting and have the same positive effect (drowning a fraud in valid results).