It's very difficult to trust an anonymous user with little track record.
Here is an idea to reduce (but not remove) the element of trust:
Next time you run such a competition, contact 4 well known and trusted SteemIt users, and arrange with them to create a 3 of 5 multisig Bitcoin wallet. You can use Copay to do this very easily. You may need to pay them all a participation fee.
You deposit an amount greater or equal to the value of the prize to the wallet. Once the competition is over, they can validate that you have fulfilled your side of the bargain, and send you the Bitcoin back. If you fail to fulfill your side, they will choose a winner themselves and send the Bitcoin to that user instead.
It is very difficult! Any guide?
ok
Okay, now I'm looking for trusted users to the competition and the deposit of money to the prize
I don't know if that Windows client is the same thing. I'm only familiar with the Android client. Sorry that I couldn't be more help, I tried searching for a guide but didn't find one.
Be careful not to be scammed yourself. I suggest creating a new topic and asking the community to nominate people who would be both trusted and willing to facilitate this. Make sure they are who they claim they are.
How to create a 3 of 5 multisig Bitcoin wallet?
As I mentioned, I'm only familiar with Android. Hopefully it is similar enough on Windows.
Click on the menu (three white bars on Android)
Select + ADD WALLET.
Select CREATE NEW WALLET
Select SHARED WALLET
Choose a name for the wallet and your own name
For TOTAL NUMBER OF COPAYERS select 5.
For REQUIRED NUMBER OF COPAYERS select 3
Then you take a screenshot of the QR code and share it with the other participants.