How should a helicopter pilot react after V-belt breaking?
A number of R22 accident reports indicate that the rotor RPM drops and the engine will tend to overspeed in case of 'V' belt failure. From the Australian Transport Safety Bureau report Reliability of the Robinson R22 helicopter belt drive system:

NTSB occurrence report: LAX92LA034
... pilot reported that the engine RPM indicator suddenly pegged at the top of the gauge and the rotor RPM began rapidly decreasing. ... Examination ... revealed that the forward v-belt was split longitudinally and displaced from the transmission pulleys. The rear belt was found off the engine and transmission pulleys.
CAA occurrence report: 99/23
... there was a bang and the engine ‘revved up’, but power to the main rotor was lost. The power loss to the main rotors was due to broken v-belts.
NTSB occurrence report: NYC08LA043
... , the engine’s RPM sharply  increased and the main rotor RPM alarm sounded. Post-accident examination of the helicopter drive system revealed that the v-belts had rolled off the sheaves
NTSB occurrence report: N993KC
The pilot ... heard a loud ‘bang’. The pilot ... observed the engine RPM was excessively high, the rotor RPM was below 80 percent and the low rotor
RPM warning horn was sounding. The on-scene investigation noted that one of the v-belts to the main rotor drive system had separated,  which caused the other belt to slip off the upper spindle.

In case of suspected belt failure, the pilot is expected to try and make a power on landing while prepared for an autorotative one. From the R-22 Flight Manual:

... if the clutch light flickers or stays on for a longer time than usual,  it can indicate a belt or bearing failure in the vee-belt  drive. If that occurs, immediately pull the  CLUTCH  circuit breaker. Select the closest  safe landing  site and make  a normal power-on landing. Be prepared  to  enter autorotation should failure of the drive system occur. The smell of burning rubber may also indicate an impending belt failure.

Note that there is no talk about the throttle. Though the R22 has two V belts, it is not designed to fly with a single one (though there seems to be some instances of that). My guess is that it is better for the pilot not to tinker with the throttle for two reasons- if the problem is somewhere else, reducing the rpm would have adverse effects and if the belt is indeed gone, it would be better for the pilot to concentrate on saving the people first and the engine later.