FrustratedWithFormsDesigner is correct, PL/pgSQL can do this. Here’s the script:
CREATE OR REPLACE FUNCTION truncate_tables(username IN VARCHAR) RETURNS void AS $$
DECLARE
statements CURSOR FOR
SELECT tablename FROM pg_tables
WHERE tableowner = username AND schemaname="public";
BEGIN
FOR stmt IN statements LOOP
EXECUTE 'TRUNCATE TABLE ' || quote_ident(stmt.tablename) || ' CASCADE;';
END LOOP;
END;
$$ LANGUAGE plpgsql;
This creates a stored function (you need to do this just once) which you can afterwards use like this:
SELECT truncate_tables('MYUSER');