alternative fix to: How can I calculate a Joule Thief
As I have seen Amstrong oscillator circuit configuration in mobil phone chargers, that could be used here too. Simply you could detect overvoltage through a opto-isolator and kill the oscillation and force transistor to cut off.

You only just need a feedback. Study these example. 


Original link to circuit,it's public on web

Please don't do modifications to this circuit , instead get the idea and do 
modifications to your joule theft design.

Hope this will fix your problem. [ Mobil Phone charger idea have been shamelessly
used in everywhere now :P ]