I just wait for the other party to hang up.
Once they have terminated the call, the homepod switches to sleep.
If you tell siri to hang up and the other party has beaten you to the punch, siri tells you that there is no call, lol
I Personally don't feel comfortable having the other party hear you tell siri to hang up.
Ido prefer using the homepod for speaker calling...good mikes, little to no hiss, no pops, bangs and screeches that occur wit my iphone on speaker.