My rats have begun to use the plastic shelves in the cage as a toilet instead of the bedding at the bottom which is making my room smell

Is there any way to dissuade them from doing this?

I have found that rats can be lazy about how far they want to travel to poo. I have two small cat-sized litterboxes in my cage. I have the larger Critter Nation so I have a litter box upstairs and one downstairs and it seems to work well for any poopy urges that come up.

What is your cage setup like - do they have to go far out of their snuggle area to poo right now?

If it's making the room smell, clean that area daily. I've found that you can't force them into pooping somewhere they don't wanna go, you just have to work around their habits.

If they're not already litter trained, perhaps give it a try

Everyday or anytime you notice poop other then where it's supposed to be just keep moving it to the right spot. It might take a week or so but rats can start to pick up on where their poo is supposed to go. Getting a litter box, even just a cardboard box you can throw away each time you clean out their cage can really help keep the poop in one area.

Hmm.. perhaps I'll try using a small cardboard box (or shoebox lid) and putting some bedding in it on the top plastic shelf and hopefully they'll use it...

And won't kick the bedding everywhere xD