Reduce Power of High frequency output signal (40khz)
On the board, I see a "220V" input connector, rectifiers, and no smoothing caps. Thus I'm going to assume this runs directly from rectified mains. There doesn't seem to be any voltage regulation, and there are only two active devices.

Also the board has "110V" and "220V" versions, which means it can't adjust its power output for lower voltage.

So... if you want to reduce power, you could try powering it from an auto-transformer (variac) to reduce mains voltage. It probably needs some minimum volage value in order to run, though.