How to limit volume on an Up2Stream Mini ? HW or SW based

I haven’t worked yet with the APC Workbench software. But I have seen, that many settings can be handled using this software.

I am building a wireless speaker system that will be used in a public place. People passing by will have the opportunity to connect to the WiFi and stream their own music to that speaker.
In this environment, I want to be able to maximize the volume of the speaker to a certain level. Especially to prevent that people can put it to 100% and annoy other people.

(1) Is it possible to use APC Workbench with an Up2Stream Mini, to maximize the volume to (for instance) 60% ?
(2) Is it also possible to limit the volume by adding hardware? (Potentiometer …)