L298 with stepper.h disable break
There is no facility in the stepper library to do what you want - however the stepper library just sequences IO pins in the right order.

There's nothing to stop you just placing those same IO pins in a different state - the library won't care.

void sleepStepper() {
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);
    digitalWrite(10, LOW);
    digitalWrite(11, LOW);
}


That will effectively set every port of the L298 LOW, which is equivalent to "brake" mode on a normal DC motor, or "not actively driving" on a stepper (not perfect freewheel, there is still some flyback braking).

Another option is to connect the EN pin(s) of your L298 chip to a GPIO and use that to turn the whole system on and off. That will disconnect the drivers from the motor and give you even less movement resistance than the "all low" method above.