question about preventing filling a survey a second time with cookie

[Eric]

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", _
" alert(already filled.');" & _
" window.location.href='http://xye.sd.sd/';", True)
Else
Response.Cookies("enq").Value = "1"
End If
....

I can see the cookie in C:\Documents and Settings\myaccount\Local
Settings\Temporary Internet Files

But i visited recently a website also with a survey. I filled it, then made
the whole Temporary Internet Files directory empty to be sure, i closed the
browser and then i went back to the same site, i tried to fill the survey a
second time, but i got the message "you have already ..." .

I even started Firefox on the same client computer and there too i couldn't
fill the survey.

My conclusion is that there must be another cookie placed in another
location.

Can someone tell me what i can do more than my code ( can i redirected
another cookie in another place?) or does it exist another way to prevent
the second attempt?

Thanks
Eric