vertical scroll - is core feature of scrollable dataTable :) Used for lazy data loading. If the data will fit to the table size - should not be shown actually.
but it showing in mozilla...rich faces version 3.2.1...and problem also present in first time column header rendering..i can't switch 3.2.2 as per project stage....
Currently supported branch is 3.3.x and latest supported release is 3.3.2-SR1. Even if we will confirm some issues - patching work should be done on your side for so old version.
And b.t.w. scrollable table has many issues fixed for 3.3.x releases, but not for 3.2.x.
And for original problem check the CSS applied in firebug and redefine it in order to hide the scroll.