How can I limit the volume on integrated amp

Small speakers. :)

I would just get a network stereo receiver and some cheap but decent small used bookshelf speakers and encourage low level listing.  Just my guess but most renters probably would not care too much about sound quality just as long as they have music.  Or just get an AVR that has max turn on volume as well as max volume settings.

Bill

Your "guests" can listen through their  Iphone. They're just there to sleep anyhow.We have Air BnB's in the neighborhood. People in and out, on a daily basis. Inconsiderate,entitled and unwanted.

Absolutely the worst idea for a business, save for the property owner and user. Ruins the fabric of the neighborhood.
Don't give them an opportunity to annoy the neighbors.

Disregard if you're not operating one of these operations, though I'm guessing you may.