You can only get the ability to clear an alert for Police or Hazard, etc., if you are traveling on the same stretch of road in the same direction the item was reported. In that case, you get a prompt asking you if it is still there and you can hit the "Not there" button.

That's good, I will try it the next time!

On the iPhone you can also try deleting from the report list.

Do a right swipe of the report, and a delete option becomes available.

I have fairly good results from this, buy my commute has about 3 wazers on it, so the reports don't have many vote ups. There have been some conclusions that their are hidden weights behind these reports, and that it might take a couple delete requests to remove them.