Dear @Jessica Fraser,
Welcome to our Community!
In order to prevent employees from requesting an absence in a certain period of time, you can create an absence type and name it, for example “no vacation during this period”. Afterward, you can import an absence period for the time you would like to block. If your employees request an absence during the blocked period, the approver will get a notification about two absences are overlapping, so they will have the chance to reject the vacation request.
Another user has asked a similar question before:
Take a look at the process explained in the post and in the help center article. If you have any question, feel free to write me on this thread again .
I wish you a lovely weekend!
Best,
Andrea