You can deactivate a user by going to the Users tab in Admin section. Search for the team member's name in your user list and click the checkmark in the box under the Active column. This will deactivate their account and not allow them to sign in to their account moving forward. This will move them to the In-Active list and their redemption/shoutout history will still be available.

Here is a quick video going through the steps on how to deactivate a user.

You can't delete a user in Nectar. This is because redemption history needs to be kept in the system. Deactivating users will prevent them from logging in, redeeming points, or performing any other action.