Hi Heidi:
I would not decay the scores below 0 - you either have a score, or you don't. And if you are a bad person, you get black listed or marketing suspended...
The first thing I would do is add a filter to your inactivity scoring campaign, which requires a minimum of 5 Lead Scoring points to run. This would prevent the score from dipping into the negatives.
Next, you have to update your existing leads with the negative scores. I would consider running a batch campaing on all leads with negative scores, and Change Score to =0.
If you want to hold on to your negative scoring values and wait until they become active again, you can create yourself a smart campaign that listens for the desired activity, and then changes the score to 0. However, this activity may be worth some extra points on top of the 0.
Your regular behavior scoring campaign may cover this, but you may run into a timing problem. You can't control, if the update campaign or the scoring campaign executes first, unless you add a one minute delay to your scoring campaigns, so that the update campaign can finsh.
Can you see how this gets messy and unnecessarily complicated?
You are much better off keeping your scores positive.
Hope this helps!