There is no easy way to do that. It will take the modification of the forum DB. Things should even out over time, and people with negative karma can change how they approach this forum and will likely raise their ranking.
Ideally, you'd only be able to "vote" once per post, in addition to the once/day rule. So, for example, someone who only has one post, could get (at most) one negative point per user who visits that post - after they've each voted on that post, they cannot add additional karma for the same posting. After all, if someone only says something once, they should only get rated for that statement once.
Of course, as far as I'm aware, that's not available in the forum software, but it would be a cool feature if it ever becomes available.
Edit: it would also be cool if there were a way to set it to add a karma point per week, as long as there are X number of posts that week (up to zero, though, not something someone can gain positive karma by), so that those who get negative karma but then stay around without additionally offending anyone, get back to neutral eventually. Sort of an electronic version of "forgive and forget." Not giving someone positive points that would imply they are trusted, but sort of like, "yeah, that guy used to be a jerk, but he's mellowed out over time, so I don't mind having him around, now." Even with that sort of system, someone who keeps posting nonsense is going to gain negative points far faster than the one/week positive point the system would add.
Again, not aware of anything that can offer that, at present. But, eventually, I hope that the karma systems receive some helpful programming attention by those with the skill and interest.