the simplest way is to just turn the knob and leave it there... lol
Or you could just take the knob out, turn it to full power and tape it up and thrown it inside the amp.
But you should be able to turn the pot to the right, remove the pot and measure the resistance, of the pot in the high power position, then get a resistor that is the same resistance and solder it it, or it may not even have resistance in the full power position, in this case just unsolder the pot and solder the wires together.
But the REAL question is WHY do you need this done?