I recently looked into purchasing a used AP for my 05 STi. Apparently the guy didnt umarry it properly. I called COBB and they will re-set it for $395. Needless to say I didn't purchase that and just didnt want the risk/hassle of buying used to I got a new one. Extremely happy with it.
The only thought I'd have to your problem is either send the ECU to Cobb (I would think they could clear it out), have the dealer re-flash it and erase the Cobb map and marrying info, or replace the ECU.
I would try to have the dealer get ahold of the guy who traded it in and see if he still has the AP to unmarry it. If you bought it from a dealer they really should work with you to take care of this.