You could do something like this. Since 15000/6 = 2500, you could hit Cntl-g
(for goto, type the range a2500:a5000 (or something very similar) hit cntl-x
(for cut,)
then right arrow end up (hopefully you will now be in b1,) and cntl-v for
paste. Repeat the process going from, for instance a5001:a7500 (or
thereabouts.) When finished doing all 6 (or 8) columns, go down to row 2500.
Hold shift and then hit cntl-end to go to the bottom of your spreadsheet,
right click and hit delete and answer entire row. Then save (this should
make it so Excel doesn't think your used range is as far down as it was
originally.)