When setting up a time off type, you have the option to only allow this to be booked by HR.
To enable this go to
Administration > HR > Time off management > Time off types
Click on 'Add' to create a new time off type or click on the 'Edit' icon to edit an existing time off type.
Scroll down to the Display Settings section and from 'Only bookable by HR', select 'Yes'
Once this option has been set to yes, no other user groups apart from HR or administrators will be able to book, edit or delete time off.
This setting will override the option named 'Bookable by employee'. If this setting is set to 'yes' and the 'Only bookable by HR' option is set to yes, 'Only bookable by HR' will take precedence.
Click here to view how to create time-off types.