As for keyboardists... a more physical approach will be necessary to keep them from going were they shouldn't ;-) .
2 months ago I told the keyboard player if they kept on using that much left hand I was leaving, and I'd slash their tires on the way out too. It seemed to get the message through as there have been no more issues with hogging the bass area.
She knew I wasn't serious, much. (I treat all musicians equally)

