Hi Tjark,
When running locally with an HSQL database, you can always view and interact with the local database from the built-in view in studio pro. (see here: https://docs.mendix.com/howto/collaboration-requirements-management/sharing-the-development-database#5-starting-the-database-viewer) This viewer also allows you to run arbitrary SQL statements, so you can delete your record this way:
-- sample delete
DELETE FROM "PUBLIC"."myfirstmodule$post" where "id" > 0
Of course, you could also just build a microflow or page that gives you access to delete the record as well :)
Thanks,
Conner