Create Custom Sandbox for EXE

I have an application that implements a .NET Browser control for accessing "offline" as well as "online" content. At times, I need to prevent the application (the browser control) from downloading elements live off the internet to render a web page that is displayed "offline". So what I need here is a sandbox that I can turn off and on at my discretion. I cannot rely on Windows Firewall since most of the time, third party AV programs turn it off by default anyway. I cannot have my users monkey around with custom filters, rules or settings in their respective AV or malware detection software. I was thinking that this could be a separate .EXE that I could run as a Process from within my C#/.Net app when required. Alternatively, I thought maybe a "Service" could work too. I'm not sure what the level of difficulty for something like this is....or whether it can be done. If you have read this project description and you are interested, please reply with the word SANDBOX somewhere in your comments. Since I cannot predict the cost involved (which I hope could be affordable), I cannot guarantee that I can proceed with the project 'now'. BUT, if the price is right and I have a sense that the project will be done properly, then we will proceed.