Changing OS X Spotlight priorities for specific applications
You don't need to delete anything. And I strongly advise you not too, since it's a built-in app. It could be useful to other part of the system, and it will probably be added back on a system update.



About your problem, Spotlight is smart, it will learn that when you type "photo" you want to open iPhoto over the time.

Just keep on typing photo, and selecting iPhoto over PhotoBooth.



If Spotlight is not smart enough or fast enough for you, you can try an alternative launcher, such as Alfred. It may be faster to learn than Spotlight (at least, it's what I found happened for me).