So actually this wont be a problem. Basically if they do this then each account that they create will only have very few published predictions. So it will be like a coin which comes up head twice after two tosses. The public will not trust such accounts with low number of correct predictions.

To build trust you'd need to have a high count of revealed predictions in any given account.

That said people can try to game the system by uploading a very large number of predictions programmatically, so I think a rate throttling will have to be enforced. Any account can't make more than one prediction a day or something like that.

Also note that the issue you raised will also affect OP's implementation.

Add cost to post the prediction. At high enough cost it can't be gamed.

edit: and it's also your revenue.