My idea is to start the student point value to -10 in the forum making them participate or lose points. I went to the grading section and set a student grade to -10 but it tells me that I cannot set it to below the minimum.
From your suggestion, I do not see a direct method to change the Maximum as a setting. Is this done my selecting a scale set up under Grades? I tried that. When I grade a comment in the Forum, for example. my scale goes from -10 to +10 but it shows a min/max range of 0 to 21. I doubt I would be having problems here as well as elsewhere if the scale was based on value rather than on ordinate position or index.