JRG has asked a question you can link to, now you can do this:

 - If you see a question referencing a random .deb from the internets, edit it and add this notice:

   > **Note**: Installing packages from untrusted sources [**may harm your computer**](http://askubuntu.com/questions/97667/what-precautions-should-i-take-with-debs-i-find-on-the-internet). Please consider finding an alternative, trusted source if you have to install this package.

 - If you know of one, definitely add a comment pointing to a *more trusted* source, such as an officially endorsed PPA (by the vendor) or the Ubuntu repositories themselves.

 - If you're sure your new source doesn't harm the quality of the answer, replace it altogether rather than adding a notice.

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Please remember, this is only an "official policy" if you actually start doing it. Until then, I merely post this for the purpose of discussion.

But I'm championing this solution. Even though a couple of users will get disgruntled at the suggestion that their package is 'harmful', in the long term it'll help keep Ubuntu secure.