Unique constraint on only some of the rows

Hello there!

If I want to create a table

create table users (
  id integer
  name varchar(8)
  enabled boolean
);

and a constraint  unique(id,name) but the unique constraint should
only be used for the enabled users, how can I do that?

The only way I can think of is to use null values when a user is not
enabled and use the constraint  unique(id,name,enable).

Are there other ways of doing this?