I wouldn't recommend interrupting it. If this is the first time it has been backed up to iCloud this can sometimes take quite a while. If you look at the top of iTunes where the progress window is, if there is an "x" to the right you could try clicking on this to stop it.
If you absolutely must stop it, don't just pull the plug out. At least eject it first by clicking the up arrow next to the name of the phone.