Is there any way yet to delete screen datablocks in outliner ? I have a couple of temp screens from my old startup file that freeze 2.75.
To delete one or several screen layouts you should use Info window:
Here the same technique as for deleting images is used, i.e with Shift pressed click button near the screen layout name to delete it completely from the .blend (it will disappear after reboot).
To delete many layouts, it may be easier to delete all of them except for one.