A Delegated Administrator is unable to remove a contact in Support portal as the user does not appear under 'Contacts with Full access' or 'Contacts with Limited access' section.
Users will appear under the 'Contacts with full access' or 'Contacts with limited access' section only when they have been granted access to support entitlements for an account.
If a user needs to be removed and is not listed in the 'Users' section of the portal, please contact support by initiating a chat session or by creating a support case here.
To remove users who have been granted access to support entitlements, please follow instructions in the article How to remove a user from an account on Support Portal.
To assign support entitlements to a user's profile, please refer the Support Portal Admin Quick Guide here.