As near as I can tell (using tools like WiFi Explorer) disabling my Comcast router's xfininti/XFINITY hotspot(s) only hides them. The antennas appear to still be on (the MAC addresses associated with the Comcast hardware still appear in my list as active, but just without a network name) resulting on overall poor quality and performance for my WiFi. Is that what's going on? If so, how do I actually turn off my xfininti/XFINITY hotspot(s) completely so that I don't get interference from them?
This is the standard for Comcast and it is part of how they can promise the roaming Comcast connectivity ("Wireless Internet on the Go"). The radio cannot be disabled, and any attempt to do so in their interface will only disable wifi for the local network.
I presume the public-facing wifi is on a VLAN or something.
The easiest way to fix this is to get your own DOCSIS device (aka modem). They have a pretty good list of supported consumer models and there are some decent ones for not a lot of money (just check to match the throughput to your plan). My daughter picked one up for about 80$, alongside another 80$ for a decent router. Compare this to the rental fee for their equipment, and it is basically a wash.