How can I delete the completed questionaire forms in my account without deleting the form itself?

To my understanding, you are referring to the submission data stored in your form. To delete form submissions, please follow the instructions in this guide — How-to-Delete-Form-Submission-Data

Note: Before you delete form submissions, make sure that it's backed up. Deleted submissions or irrecoverable.

How-to-Partially-Download-Form-Submissions-as-Excel-CSV-PDF

How-to-download-all-or-partially-your-submissions-file-uploads