One of the knobs on my amp is broken and it's not just the plastic, it's actually the metal as well.  I think this is the cause to my random volume spikes on the amp.  How can I disable this knob or at least turn it down to 0 so it doesn't affect my normal sound?  Thanks  Here are some pictures of the amp itself.  It has random spikes of volume.  I was told to try to turn the knobs to extremes to clean the pots.  Any help on the knob though?   Greatly appreciated!

Pictures: Link