Issue an order to domesticate them.
Not the feminists or melanin enriched folks.
It is very common and popular among hipsters to keep a rat as a pet. Some people told me they have two or more.
Yes, I know. Those domestic rats are not smelly sewer creatures. They are playful and smart. Sure, they poop everywhere and chew everything, but this is easily forgivable. Still, domestic rats are sort of middle-class compared to proletarian sewer creatures.
But why domestic rats are so nice? Because they are socialized. Domesticity is a social construct. Even the most Right-wing minds would agree with that premise.
Sewer rats are simply maligned for too long, and blamed for every disease, while the domestic ones are pampered. That is the only real hardcore difference between them. Genetically they are nearly identical and moreover, all genetic differences may be explained by domestication.
So, my proposition is an enforced and sped-up domestication of all rats. Sort of busing for ratty sewer youths in order to desegregate them from the middle-class pets. Wide-range positive social engineering and affirmative action for sneaky, bitey adolescents. Offer them domestication scholarships.
Sure, most sewer dwellers would rather choose to live in a clean apartment while undergoing aggression reduction training. For each time they don't bite the hand that feeds them, give them an extra treat and luxurious lifestyle. Soon they will turn into propagandists, go into sewers and convince the rest of rodents to join in.
Rats respond well to incentives. Give more incentives to female rats, and male rats would also try and copy their submissive mating behavior patterns and may even begin to practice alternative sexual roles, greatly reducing the birth rate.
Issue quotas - for every ten rats purchased in pet shops, there have to be at least 30% young sewer-born ones right now and 51% in ten years.
Produce more movies reinforcing the positive image of a sewer rat. Ratatouille is but one small step in the proper direction. Also all the bigoted negativity must be eradicated and diversity among pets to be celebrated non-stop. Which means domestic rats together with chihuahuas and cats should give their privilege away and accept their lesser position in the world of tomorrow. Imagine the world without cages or sewer covers and keep your disbelief suspended.
And with a bit of extra funding we may clean the city sewers and turn them into high-rise apartments for the growing rat population.
This is the only way the justice may be done to these wonderful mammals, our not so distant relatives on the Tree of Evolution.