# /etc/pulse/default.pa
ifexists module-bluetooth-policy.so
  load-module module-bluetooth-policy auto_switch=false  <-- add auto switch
.endif

.ifexists module-bluetooth-discover.so
  load-module module-bluetooth-discover
  load-module module-switch-on-connect                   <-- add this
.endif

# /etc/bluetooth/main.conf
[General]
Disable=Headset