To delete a survey you first need ensure that it is not attached to any document or else the users will receive an error if they try to complete it via the document (i.e. when they select “Complete Survey” on the document library Ribbon or when they try to complete a DocRead task assigned to a document that has the Survey attached).

To delete the actual survey, go to the DocRead “Manage Survey” page (in site settings) and select the “Survey Settings” option from the survey drop-down menu.
From the settings page select the “Delete this Survey” option.

This will delete the survey but it will not delete the statistical information for that Survey.

The statistical information for a survey cannot be deleted, it can only be archived via the “Archive Statistics” option.