The process cannot acces the file 'ssisproject.ispac' because it is being used by another process.

Case
I want to run an SSIS 2012 package but I'm getting an error:

ispac file in use by other process

System.IO.IOException: The process cannot access the file 'c:\folder\ssisproject.ispac' because it is being used by another process.
     at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
     at System.IO.File.Delete(String path)
     at Microsoft.DataTransformationServices.Project.DataTransformationsProjectBuilder.IncrementalBuildThroughObj(IOutputWindow outputWindow)
     at Microsoft.DataTransformationServices.Project.DataTransformationsProjectBuilder.BuildIncremental(IOutputWindow outputWindow)

Solution
You have probably already restarted SSDT, but that didn't help. Open the Task Manager (Ctrl + Shift + Esc) and locate DtsDebugHost.exe under Processes and end that process (could be multiple times, end them all). Now try running the package again.

Task Manager, End SSIS Debug Host

This happens when SSDT/Visual Studio crashes during runtime. If you kill SSDT then the SSIS Debug Host will still be active locking the ISPAC file.