You cannot stop it you have to prevent it.
Once rats learn to chew they will keep chewing no matter what even if place in another enclosure, worse they will teach others to chew as well, the only way to stop it is to feed the chewers off and by that I mean anyone in this enclosure.
When I have chewers and I rarely do they get retired IMMEDIATELY (no second chance).
Now how do I prevent chewing? I provide a wood block to all my rats, it's been working very well for me.