The only time I've had a sink full of dirty dishes complete with rotting food and flies is when I was spitefully waiting for my roommate to clean them up because it was "his turn".
It won't result in them washing the whole pile, but they will be forced to wash something and realise that it doesn't hurt that much.
There will still be a pile of dirty plates (plus the boxes from the pizzas they ordered for the first week, until they ran out of cash) when you get back, but at least you will see a slight improvement in your roommate's habits.