Pacific Timesheet allows you to create timesheet templates that only allow leave or time-off to be placed on them. If you have a group of employees that are only required to record leave, then all you have to do is go to their timesheet template and edit the advanced settings to only allow leave time entries:

When you make this change the employees using this template will no longer have an 'Add Work' button on their timesheets, only an 'Add Leave' button. One other behavior that changes is that, by default, these employees will not receive timesheet due/late notices if they haven't submitted their current timesheet. This is because employees would usually only submit a timesheet if they took leave in that time period. You can change this email notice behavior on the System > Notices page.