I am looking for the best way to unmonitor a device without deleting it.

As well, the goal of avoiding charges is impossible -- any device in the system is charged (unlike some other products that exclude down devices from billing, like Auvik). I can appreciate why this is true in LM since the concept of down can be a bit fuzzy.  I personally have never used auto-add for netscans since I am not a fan of unmanaged charge increases :).  I did write an API script to report on netscan finding details regularly (at the time that was not possible in LM, perhaps it is now).