Go to 'Online Family Tree > Configure > Privacy':

Your 'Living Individuals' privacy option must have been set to 'Semi-Public' or 'Private':

Select the 'Withheld Individuals' tab then click 'Add an individual':

Enter the first letters of the surname/first name of the individual to withhold then click the 'Add' button at the bottom of the list:

The new withheld individual is now listed.

You can remove an individual from the list at any time by clicking the 'Delete' button.

Select the 'Advanced Options' tab to change the number of years that define if individuals in your family tree are said to be alive or not.