Starting with 2005, VS started this behavior of when starting debugging session it spawns up a webserver for every project in a solution. I have a solution with 15 projects
I know this is an old question, but in Visual Studio 2010 choosing properties from a web project brings you to the big configuration screen/grid. The Always Start When Debugging
setting is buried a bit.
With focus on the desired web project look at the properties window/tab (CTRL+W, P)and set the property there.