How to discharge smoothing capacitors?
Appropriate bleeder resistors are the usual solution. They aren't usually switched, although they can be.

The value depends on the time you require to discharge the capacitors. The formula is

$$ V_{t} = V_{0} \, e^{ -t / RC } $$

where \$V_{t}\$ is the voltage at time t and \$V_{0}\$ is the initial voltage at time 0. It's an exponential function, so I'd just assume 1/10 of the initial voltage.

It isn't a power function, as someone edited it!

You should find that the power taken by the bleeder resistors is negligible compared to the 120W capability of the supply.