There are two level of access :-

1- Page Level

2- Data Level

We can easily protect user from accessing particular page through asp.net authentication and authorization mechanisim. For details see this link. Also you can implement your own Session based protection model as mentioned in this link.

As far data access, most common issue is tempring the Query string as you mentioend. To avoid that you can implement some query string encryption module through which you can assure that querystring is not tempered. Here is a sample for that which will give you a quick start:-

http://www.devwebpro.co.uk/devwebprouk-46-20070125ASPNETHttpModuleforQueryStringEncryption.html

http://ryanscook.com/adminsBlog/2005/05/querystring-encryptor-httpmodule.html

Also try to avoid using Querystring for sensitive data and use session which is more secure.