We're using Greenhopper 6.1.6 with a Scrum board. It often happens to us that someone logs work and makes a huge mistake - e.g., logging days instead of hours (because the default time unit seems to be days). When correcting the mistake, the remaining estimate is not fixed in the burn-down chart.
Here's an example:

And here's the culprit:
These spikes can get so high that the rest of the burndown is no longer readable:

What can we do to get rid of such spikes?