Deactivating an individual in your account will mean you can no longer include them in surveys and if they had a login, this will no longer work. Deactivating rather than deleting means their historic results are maintained in any surveys they've taken part in.
To deactivate a user, follow these steps:
Click on the People menu.
Search for the individual.
Click Edit next to their name.
Next to Status, select the Inactive box.
Click Save. Under the Active heading, there should now be a X to show they are no longer an active employee/user.