If an employee plans to leave the company for a period of time (such as for parental leave or a sabbatical), you have the ability to make them an Inactive employee by clicking 'Edit' on their profile, and changing their status to 'Inactive'.
By doing this you will stop receiving alerts whilst they are not at work, and they will not have access to their Engage accounts during this time.
When they return to work, you can find their profile by changing the drop-down on the Employee listing screen to Inactive, Editing their profile, and resetting their status to 'Active', including alter their Start Date / Role Start date as per above. From there, the employee will be able to sign back in, and you will continue to receive notifications for this employee as per normal.