Removing Instruments

Click the blue squarish thing at the top right the pages and delete them. You may be able to right click one of them and chose select->all similar items and then you can delete them all at once. I believe there is a bug that I haven't verified yet that may also remove other breaks, such a section breaks - which might be in the score and line breaks - which shouldn't be in that score.

Depending on your reason for wanting to do this, it might make more sense to simply extract the parts using File / Parts, then you'll get a separate part for each instrument, all formatted appropriately. If you start from the score and delete instruments one by one, there may be various other things in addition to the breaks you'd need to adjust manually in order to get something reasonable. Eg, enabling multimeasure rests, perhaps changing page and staff size, adding instrument name to the top left...