<p>I know you can do it file by file.</p>

<p>Is there any way to do this in one step for all files in a project?</p>

Do you mean using statements? First, note that they generally do no harm other that take space.
Tools like
ReSharper
offer automated tricks to do this, however: there was a
link in the VS feed
a little while ago; it boils down to:
go to Tools -> Macros -> Macros IDE...
in the Project Explorer, Add -> Add Module... (put in a name - I've used OrganiseUsings)
paste over with the code below
File -> Save MyMacros, exit
Now if you right-click on the toolbar and Customize... - you should be able to find MyMacros.OrganiseUsings.RemoveAndSortAll - drag this somewhere handy (maybe the Tools menu; you might also want to change the name after placing it).
You can now use this option to run the Remove and Sort command for an entire solution. A big time-saver.
==== code ====
```
```
Imports System
Imports EnvDTE
Imports EnvDTE80
Imports EnvDTE90
Imports System.Diagnostics

Public Module OrganiseUsings

    Public Sub RemoveAndSortAll()
        On Error Resume Next
        Dim sol As Solution = DTE.Solution

        For i As Integer = 1 To sol.Projects.Count    
            Dim proj As Project = sol.Projects.Item(i)    
            For j As Integer = 1 To proj.ProjectItems.Count    
                RemoveAndSortSome(proj.ProjectItems.Item(j))    
            Next    
        Next    
    End Sub    

    Private Sub RemoveAndSortSome(ByVal projectItem As ProjectItem)
        On Error Resume Next
        If projectItem.Kind = Constants.vsProjectItemKindPhysicalFile Then    
            If projectItem.Name.LastIndexOf(".cs") = projectItem.Name.Length - 3 Then
                Dim window As Window = projectItem.Open(Constants.vsViewKindCode)

                window.Activate()

                projectItem.Document.DTE.ExecuteCommand("Edit.RemoveAndSort")

                window.Close(vsSaveChanges.vsSaveChangesYes)
            End If    
        End If    

        For i As Integer = 1 To projectItem.ProjectItems.Count    
            RemoveAndSortSome(projectItem.ProjectItems.Item(i))    
        Next
    End Sub   

End Module
```
```