Transaction Deleter

Take a backup.
Compare with the original Chart of Accounts you started with.
Cherry pick the differences in the former you wish to retain and then what is left can be restored to be the one without any transactions.

Alternatively, compare the en_US-new.sql and the en_US-demo.sql files in the sql folder and see what transaction tables need to be purged and then purge those tables in your database.

@joe: Though dangerous, this can be an useful menu option greyed out by default and with a permission setting to be enabled on demand atleast for the Super Administrator role.