Hello Andrea,
There are different ways of setting up security in BO. So it is not possible to explain completely here.
However please try below. It below might help you.
Go to user security of a folder which you want to hide.
Select the usergroup >> Assign Security >> 'Advanced' tab >> Add/remove Rights
[use advanced rights to explicitly deny a user or group]
From the list of rights for "view object" Set to 'Deny'.
Also, you can play with various options like 'Apply to object' and 'Apply to sub object'.
Good Luck
-Mahesh