The easiest way to deal with busking problems is to a) only give permits after an audition, and b) forbid amplifiers and loudspeakers. Add a "you may only stay in one place this long", and it should be no problem anymore.
Well, I imagine, it's more a matter of them needing an amp, because they're playing an e-guitar, and once you have an amp, might as well turn it up. Playing the bagpipes or a trumpet/trombone/whatever loudly is actually exhausting...