Well ... 

I don't suppose you told her at the beginning 
that any and all payments would be forfeitted if 
she stopped making payment?  I know there are 
lots of people do that (myself included) when 
they accept payment plans.  It puts the pressure 
on the buyers to follow through.  

Other than that, honestly, the only way to get 
her completely out of your hair is to refund the 
money and tell her that you're keeping 10% for 
restocking (and for holding you up for 4 
months).  Because if you give her the less 
expensive cel, she'll keep pestering you and if 
you give her the most expensive one you'll feel 
duped.

This is a really tough situation.  Good luck with 
it.