i already said fire would do the job to cleanse all rats.
No it wouldn't, you're severely underestimating how frequent they are. They'll be like several hundred generations per week (gross understatement) while people all around the globe coordinate this fire plan.
so, a big fire would took them all out. you are underrestimaiting the power of it.
You are overestimating and underestimating the power of a fire, underestimating the intelligence of rats and other rodents, and underestimating how much you would have to burn.
To kill ALL rats and other rodents using fire, you would have to destroy every single human habitat on the planet, every single tree, every single field...
If you miss just TWO rats, you have half a billion on your hands in just 3 years time.
3 years of time is long enough for science to think of a way to kill them all beside fire. so, just use fire to kill those that are close first, then, there would be enough time to think of another better way to do it.