If you want to delete single participants' data, e.g. from a test run, you can delete them from the single results.

Please proceed as follows, to delete one or more participants from the results:

Navigate to the single results of your survey
Select the desired participant(s) through the checkbox
Select the checkbox at the bottom of the site as well and confirm the deletion

Note
After deleting a participant the corresponding data will no longer be shown in the results, but still be counted for the maximum number of participants. You can see the participants' count on the settings page of the survey. At the 'Status' the total number of participants at that time is being displayed to the right.