Moodle provides default negative grades but actually they are not counted in final results.Here is a patch to employ negative marking to your questions.It is only for multichoice questions.Just follow the instructions.

Instructions:
1)First take a backup of these two files:
  a)File: moodle\lib\questionlib.php
  b)File: moodle\question\type\multichoice\questiontype.php
2)Now replace these two files with that one provided in question folder according to their location.

It works greatly with Moodle 1.6 and later upto Moodle 1.9.x

by nCube