If all the documents are in the same space, and the space contains only documents that belong to only one form, the most direct option would be to delete all the documents in the space.
If the documents are in different spaces or the space containing them has documents from other forms, the documents will have to be deleted in batches. This can be done from the Form Documents tab, selecting the documents and clicking on Remove.
You can select multiple documents by clicking on the first and last of the group
of documents to be selected while holding down the SHIFT key.
This same operation can be carried out from the search, but first, you must filter the documents by a form.