How to stop iPhone from listing iMac as audio source during phone call?
On your iPhone, go to Settings > Phone > Calls on Other Devices > un-check the Mac. Then switch off the "Allow Calls on Other Devices" toggle switch and restart the phone. 

Also worth mentioning: simply switching off the "Allow Calls on Other Devices" toggle switch does NOT solve the problem - the Macs remain in the audio options list even if that option is off. You need to leave the option on and un-check each device before toggling the option off.  You may not see the audio choices change until the phone is restarted.