Well, you are a gentle person and I will respect that.
With that in mind, here is a quote from the Humane Society:
"Lethal control
There are no truly humane ways to kill rodents, only methods that are less inhumane. Rats are killed with poisons, snap traps, glue boards, and maze-type traps that drown them. Based on what is known about these methods, the traditional snap trap, and perhaps the newer traps that use an electrical charge to stun and kill, seems to be the least inhumane. This doesn’t mean that rats won’t suffer in these traps—they almost certainly will.
With the use of lethal control, animals will suffer. So keep in mind, the need to control rats is a largely the result of lack of cleanliness in the immediate environment."
This said, seems your only choice is to make their habitat undesirable.
With that in mind, here is a quote from the Humane Society:
"Lethal control
There are no truly humane ways to kill rodents, only methods that are less inhumane. Rats are killed with poisons, snap traps, glue boards, and maze-type traps that drown them. Based on what is known about these methods, the traditional snap trap, and perhaps the newer traps that use an electrical charge to stun and kill, seems to be the least inhumane. This doesn’t mean that rats won’t suffer in these traps—they almost certainly will.
With the use of lethal control, animals will suffer. So keep in mind, the need to control rats is a largely the result of lack of cleanliness in the immediate environment."
This said, seems your only choice is to make their habitat undesirable.