First of all, get used to unfair ratings. It sucks but this just happens, depending on how big the application it could happen a lot.
If someone doesn't like the color, not enough warnings, too much warnings, etc etc. Theres always a reason for someone to give a bad rating.
What to do about it?
Make a great app and get more and more better ratings.
What I found on your topic of removing ratings when uninstalling:
Uninstalling the reviewed app does not seem to remove one's review. (Which makes sense, I guess.)
Here is some extensive information about it: How do I remove a review I've written from the Android Market app?
Google doesn't seems to allow to remove reviews at all: Issue 16483: Android Market doesn't allow removing an existing application's rating and comment.
Tips:
Make sure you warn people in application description that you require dropbox. Not just warn in-app. This might also be the reason for low ratings because people do not expect it.
If the user would like so, cancel the purchase / refund anyway. This might give incentive for the user to change his rating before uninstalling. And gives you a good name.