Juvenile
'til proven sinful
Track them down.Here is an interesting dilemma, for those who are still online instead of driving home from work or what have you.
What would you do if a friend borrowed money from you (in the region of 10k) and refuses to pay it back or return sms' or phone calls in connection with it?
Find the friend.
Confront the friend.
Only reasonable method I can think of.