How do I stop buisness units seeing otherbusiness units contacts, accounts etc?

Within my organisation i have 3 distinct brands and i want the information for each brand to remain limited to that brand with in crm.

I have created my three business units Brand A, Brand B and Brand C but the users of the brands can still see all the information that is recorded in CRM.

How can i limit the team members information to just that whihc is relevant to their team? for example Brand A can only see accounts and contacts that are related to that buisness unit and its team members?

I have looked at the security roles as a way of limiting it but not been able to figure out whihc options i need to select to do this?

Any help wouldbe greatly appreciated? 

Thanks

Stephen