I have not noticed this before, but there does not appear to be a way to delete a single Student's answers in a Survey - as there is with a Quiz for example.
I see that the survey types on the settings page are once, daily, weekly, monthly, and unlimited. That leads me to believe that if you choose anything other than "once" then students can submit the survey multiple times. I set a survey to "weekly." When I view the responses, I can see multiple attempts from the same user. If I click on any given attempt, the date of that attempt is shown. There is an option to delete any of the responses. If a response is deleted, the student can re-submit for the week in which that response was removed. This is all possible on Moodle 2.5