In my database table, 16 rows are inserted with null values for all columns. How can I delete these rows using SQL statement?

Something like this:

DELETE FROM my_table WHERE col1 IS NULL AND col2 IS NULL AND col3 IS NULL AND col4 IS NULL

To stop it from happening in the future, consider adding a primary key to the table. By definition a PK is not nullable and will help identify the records when they are inserted. For more information on primary keys see this W3Schools page on the subject.