To delete an individual respondent you need to access the Follow-Up options: From the "My Surveys" page, scroll the mouse over the survey in question, a "Follow-up" link will appear below the survey title, click on it.
A list of respondents will appear. To delete an individual, check the box associated with that individual, then click on the "Delete respondent(s)" button.
You will be asked to confirm that you want to permanently delete this (or these) respondents. Click "Delete" to confirm.
Note: Respondents once deleted cannot be recovered. The data is lost and will not be included in analysis or reports.