You can choose between two options to delete results:

1) You could delete the complete survey. All collected results of the survey will be deleted, too.
2) You could delete single results one by one through the single results page.

The second possibility is applicable when you want to delete the test results.

Please proceed as follows, to delete individual participants from the results:

Navigate to the evaluation of your survey
Click on Single result
Select the individual participants through the checkbox
Confirm the deletion desire through the checkbox at the end of the page by 'selected participants from the results' and delete

Note
When deleting a participant, the corresponding data will no longer be available among the results. However, he will still be counted for the possible maximum number of participants. You can find this figure in the survey settings. At the 'Status', the total number of participants is being displayed.