Keep in mind that blocking out a date means you're not able to help out on ANY TEAMS. If, instead, you just want to notify a single team that you cannot serve with them, just notify the team leader.
Open the side menu on the left by swiping left-to-right, or by clicking the menu icon at the top-left of the screen.
At the bottom of the menu, find and click the + Blockout button.
From there, it's like entering something in your calendar.