You started the competition, therefore it is your responsibility to ensure that the conditions of the competition are met.
As I understand it, their were conditions that the competitors had to meet (ie., whatever the 'challenge' or contest was). The competitors who won have fulfilled those conditions. They have "kept up their end of the deal".
There were also conditions that you, the competition creator, had to meet. One of those was supplying prizes to the winners. So far, you have failed to do this. You have not "kept up your end of the deal". You have an obligation to the winners to provide them with their prize.
It's unhelpful to act as if you have no obligation to fulfill your side of the bargain when the competitors entered the competition in good faith and fulfilled their side of the bargain. They do not, as you put it, have a "right" to their prize; you, however, do have an obligation to provide them with their prize.
This was a competition after all, and not a charity. You gained something from this transaction - a warm fuzzy feeling, goodwill and respect from other forum members, amusment, a power trip, whatever - but so far, the other participants in your competition have gained nothing.
The ethical thing to do would be to take responsibility for the competition that you started and find a way to fulfill your obligations to the competitors.
In terms of actually fulfilling your obligations, the most ethical thing to do would be to consult with the competition winners in private. This is, after all, a matter between you and the winners of the competition and no-one else.
My recommendation is that this thread be closed and all interested parties be instructed to settle the matter privately.