How do you remove old alerts in 10G Grid Control?

I had an alert about a file system two weeks ago, saying 4% free in a domain. Since rectified the issue, now there's 30% free, i.e. well outside the default thresholds for the metric. But it still shows up as an alert. How the hell do I get rid of this?!?!

There's nothing in the documentation or on metalink.

Please could someone respond to this asap.

Thanks in advance.

Barry