question about using cookie for preventing filling survey second time

Hi,

I organise surveys on internet. So i send a cookie to the clientcomputer as check and avoiding a second attempt on the same survey.
I know this is not 100% sure (one can delete the cookie or disable javascript).
The code is here below and works:

Protected Sub submit_Click(ByVal sender As Object, ByVal e As
System.EventArgs)

               If Not Request.Cookies("enq") Is Nothing Then
                Dim enq As String
                enq = Server.HtmlEncode(Request.Cookies("enq").Value)
                Page.ClientScript.RegisterClientScriptBlock(Me.GetType(), "myscript", _
   &n ...