Enter your score into Sibelius with all staffs and rests present. It is essential that your score be laid out fully before you begin to remove elements.
Select your entire score. Open the "Layout" drop-down menu, and scroll down to "Format;" select "Lock Format."
Open your art program, and create a simple white rectangle without a border. If you do not have an art program, it is easy to find an image online that meets the specifications. Once you have your image, format it as a TIFF file so you can import it into Sibelius.
In Sibelius, bring up the "Create" drop-down menu, and scroll down to "Graphic." You will be prompted to select a file; choose your rectangle TIFF. Your image will now be in your score window.
Drag your image over the portions of the score you wish to remove, and resize it using the handle on the bottom-right corner until it completely obscures the staff. Import more images as needed until you have covered every part of the score you wish to cut out.