Never let the biggest ego near the sound system.

I once spent an excruciating evening watching a four piece band being totally ruined by the biggest ego. He was the lead guitar/singer and was next to the mixing desk. Throughout the evening he would tweak the knobs until all you could hear was his guitar and voice. The other guitar might just as well have not been there and even the drummer was having difficulty. The only good thing was he drowned out the melodeon.

Have a separate person doing the mixing, or else have the box as far away as possible so that no-one can sneak over and play with the balance.

If you play accoustically and have a showboater, then I'm afraid a gentle 'talking to' is needed or else a restructure and restrictions.

LTS