My ex wife's mum had this problem with her 2 girls - the older one used to chomp on the other's arm.
She tried the naughty spot, the explanation that it hurts and is not nice to do to someone, withdrawal of a few treats as punishment...nothing worked.
Final solution....her mum told her that it will an eye for an eye and if she does it again then mum will put her choppers on the biter's arm to demonstrate the pain involved...it did not happen again.
Possibly not the most enlightened response in this day and age but it did work.
If you find a solution please post as my 18mth is getting a good set of choppers and it will be an issue for us soon.
