Xcode 4 disable snapshots creation on refactoring
Solution 1:
File->Project Settings
(or File->Workspace Settings
if you're in a Workspace), and then click on the 'Snapshots' tab: you want to uncheck the "Create snapshot of project before mass-editing operations" box.
Thanks to Teknogrebo for pointing out in the comments how to do this for workspaces rather than projects.