VS 2008 Registration Form - Creating Expiring Cache

So I'm attempting to limit user's access to my database through the web. I have a registration form that asks the user for a name and password, then verifies this against a DB. In order to prevent DB overload, I will limit the user's attempts to 10 in 10 minutes. This is how I am trying to solve this, but cannot verify that i'm using the right approach.

Code:
Dim timeout As Integer = ConfigurationManager.AppSettings("timeout")
Dim userIPkey As String = Me.Context.Request.ServerVariables("REMOTE_ADDR")
Dim i As Integer
If HttpContext.Current.Cache(userIPkey) = Nothing Then
i = 1

[Code] .....

For some reason I can hit the service with a for loop and get to the DB every time. If I try doing this through the browser while debugging the webpage, I get the right error message after 10 attempts, while on the server, or through a reference to localhost I will not get the error message.