It sounds like you want the fabled “receive only” folder type which doesn’t exist. Syncthing devices are created equal and don’t form master-slave relationships. The maxConflicts setting doesn’t affect this, only how many conflict copies are kept around. However, if this:
is a one time need (the old data is there now, you want it cleared out when you set it up), just set the other device to “send only” and hit override once, once the other devices have fully scanned the old data etc.