How do I remove a project configuration in Visual Studio 2008?

In the Configuration Manager, select "Edit..." in the "Configuration" column for each project (not via the dropdown named Active solution configuration) that has configurations you want to remove.

In the dialog that pops up, mark each unwanted configuration and select "Remove".

The best way to automate the removal a configuration from all the projects of a solution is done by using Nuget console command to access visual studio apis.

Go to Tools, Nuget Package Manager, Package Manager Console.

From there use:

Get-Project -All | Foreach { $_.ConfigurationManager.DeleteConfigurationRow("Release") }

In this way you have removed all the configurations from all the projects called "Release". I strongly suggest you to always check the differences on your source code versioning sistem, you will see only csproj and in some cases sln files affected, if you are using configuration transformations (like Web.Release.config) they will still be there.

Further information are available on the visual studio version-specific api documentation here, this works from at least VS 2015 for C++, C#, F#, VB languages.