Reduce "Content Sharing" scope. Actually an user can choose the followingscopes of content:
a) The current Site
b) The global Site
c) Page scopes (if they exist)
d) Parent Site (if the site has a parent site)
e) Child Sites which have sharing enabled (if the site has children sites)
f) Any Other site in the portal that the current user can manage
Content sharing should only be possible between parent sites and its children sites which have it enabled, option f should be removed.