Are you talking about spreadsheets located on a shared network drive or a personal network drive share or on their local pc? For network drive shares access can be limited using security permission levels. Give users access as read-only to folders they should not be allowed to modify. Give only those who need it modify permissions. For a personal share only the user will have access to his own things stored there. For a local pc only users with permissions can access the file. Where are you experiencing problems exactly?