I have a Global Moderator who has been given restricted access (Member control) in the ACP. He has just informed me that he has access to the Block Manager. Is there a setting I can change, restricting other staff from access to this feature? 

ACP > Members > Staff > Moderators > Group (you wish to edit) > un-check can manage sidebar

Uncheck "Can Manage Sidebar"

Thank you both, very much.