You can choose when to reveal the results of a competition to players. By default, competitions are visible to players but you can hide a competition if you prefer.
From the Admin > Competitions index screen, click on the icon that looks like an eye on the competition you want to hide.
In the window that appears, confirm that you want to hide this competition. Once you do so, it will continue to calculate results but will not be visible to players.
To make a competition visible again, follow the same procedure.