I find it hard to believe that Canon will NOT clean your sensor!!!!  Unless there is someting more wrong than just dust!!!!
I would try again with another Canon service center....failing that, before you go and buy another camera, I would try to do a "wet" cleaning yourself....you have nothing to lose.

Wet cleaning is done by most pro's to get rid of stubborn dust or oil from the sensor's filter.

I don't have much time now....but when I have a chance, I will try to find you a link to wet cleaning techniques.

In youth, we learn....with age, we understand.