We usually recommend blocking off the guides for those times. It's easiest to do it one Guide at a time. Then work your way down the Guide list from the Timeline view. This is a great option so it keeps the rest of your guides' availability accurate.
Click Company, then Users.
Click on the calendar icon next to the Guide you want to edit.
Click Add Blocked Time.
Select the Companies the blocked time applies to (e.g. dates of trips they're already guiding), enter the start and end dates, select the start and end times, and choose blocked days.
Click Save.
Check their calendar below to make sure the blocks were added.
Note: Make sure you're not inadvertently blocking out dates/times your Guide is available to lead a trip. Double-check the calendar that you only blocked them for trips you booked over the phone.