I'm afraid that at this point it's complicated to fix things: I didn't know this IRV system and many users probably didn't know it either, I actually had to look it up on Wikipedia to see what it was. And since the rules simply ask you to vote up to 5 pizzas, that is what people will do.
I don't think that its so complicated to fix it as all that is needed is for icopress to just add a short explanation and then people can simply edit their votes.