First, make sure that they are not an owner on an active Personal Capital Cash account. You will need to close the account by following the steps here: How do I close a Joint Account?
Next, in your Settings, scroll down to Additional Users. Locate the user you wish to remove and select "Remove". This will remove their access to your dashboard but will not remove them from your profile (for example, if they were included in your retirement planner, they will still be included there). You can remove users from your profile under your Household in your Profile settings.