Then you should announce it clear that you will not give a btc rewards but its equivalent only on tokens value.
Change it to USD instead of bitcoin value if you are not willing to give btc for prizes participants will be confused to what the rewards you want to give if you will continue using btc. Some one already tagged you for that so better fix it now.
The competition is announced. The terms are published. If you don't like it, don't participate. It's so easy.