This is one of my favorite ex hacks. It's pretty old school and should work all the way back to the original Vi. This assumes your comment character is # (octothorpe), but would work just as well with // comments or any other line-wise comment character. Go to column zero of the first line you want to start your comment block on. Hit ma to set a mark. Now scroll down to column zero of the last line you want to comment out. Issue the following ex command:
:'a, . s/^/# /
That's it. It runs a substitute on the range from the mark stored in register 'a to the current line. Take note that you can specify ranges for any ex command using the same technique.