If you block dates in your calendar, volunteers won't be able to apply to them.
In order to block specific dates, click on "Calendar" on the top of the page. You'll be able to see all your positions.
Choose a position and click on "Unavailable dates". Set up the dates you don't want to host any volunteer.
You can add as many unavailable periods as you want by clicking on "Add another date".
Click on "Next" to save.
Please note you can't edit the availability of all positions at once. You need to edit them separately.