How to stop my phone from syncing to another phone after it has already started to an unknown device
Thanks for the A2A: How do I stop my phone from syncing to another phone after it has already started to an unknown device?
You do not state your phone brand, type, model ...not even its operating system... and neither the other phone's... So, this is an answer based on drastic severe approaches that for sure will work:
a) Turn off completely your phone. This will probably stop the sync process.
b) Turn off completely the other phone, for the very same reason.
c) Turn both phones off. In all options above let them turned off long enough time, so that the sync process fail and end after a timeout.
d) Since we presume both phones are syncing through some data communication media, then if that happens to be your WiFI network Turn that network's routers/modems/hotspots., etc. off. Same reasoning as above.
e) If the two phones are syncing via Bluetooth, then if you can not turn them off... Then move them far apart from each other so they can't communicate with each other.