From the Gear Menu, go to Blackout Request-Off Dates:
Choose the Departments that the block applies to:
Choose dates to block, and press Add Range
NOTE: The Block Action dropdown allows you to Allow Requests with Comments, (employees will be met with a warning that the time period is blocked; they are still able to request, but the request will be tagged), OR, not allow requests at all.
If you want requests to always be at least 14-Days out, you can choose the Block requests less than option, and add a range: