How can I restrict visibility of assets in a public communit

You have to do it by Asset type or Categorization.

You'll have to state that assets of a certain type or a certain
categorization are public, and all others are private.

On the role configuration page make sure you select the "At least one of
the following constraints" radio button. That way it will "or" them
together (any one of them will be sufficient to allow access). If you
left it as the default then they will be "and" together, in which case
they must all be assigned to an asset before it would become visible.

Then create a new role that is your Public Asset Consumer role (you can
duplicate the Asset Consumer role) and restrict it to the give asset
type(s) or categorizations. Then give signed in users that role instead
of Asset Consumer role.

--
Rich Kulp
Rational Asset Manager developer

... or, have a state (in a lifecycle) that overrides the default community permission.