Limit the volume of one speaker in a HEOS Multiroom group

I have a Denon Home 150 NV and a AVR-X2800H. I would like to either lock the volume or apply a maximum volume level to the Home 150 NV. How do I do that? I have found the individual speaker volume adjustments, but there is no way to lock the volume. If I raise the volume of my AVR, the speaker in the other room raises its volume massively to match the TV volume.