Can a PostgreSQL Trigger be used to ignore an insert?
Yes, use a BEFORE INSERT trigger, and return null.

Live example here.

create table stuff (
  stuff_id int primary key,
  thing text
);

create or replace function stuff_inserting() returns trigger language plpgsql as $$
begin

  return null;

end $$;

insert into stuff values (1, 'asdf');

select * from stuff; /* returns 1 row */

create trigger inserting before insert on stuff for each row execute procedure stuff_inserting();

insert into stuff values (2, 'fdsa');

select * from stuff; /* still returns only 1 row */