Put your cursor on the first
# character, press Ctrl V (or Ctrl Q for gVim), and go down until the last commented line and press
x, that will delete all the
# characters vertically.
For commenting a block of text is almost the same: First, go to the first line you want to comment, press Ctrl V, and select until the last line. Second, press Shift
# Esc (then give it a second), and it will insert a
# character on all selected lines. For the stripped-down version of vim shipped with debian/ubuntu by default, type
: s/^/# in the second step instead.