There are a couple ways this is already done.

1. When a question's score drops to -4 (or below) it is removed from the [front page](http://mathoverflow.net/), and is therefore "hidden" from many/most viewers. In this case the question still exists undeleted, and will be shown if you view questions from another list (_e.g._, tag-specific lists, other search results, and even the ["questions" tab](http://mathoverflow.net/questions)).

2. Closed (not as a duplicate, and unlocked) questions with non-positive score and no upvoted or accepted answers are automatically removed if they have been "inactive" for 9 days. There are other forms of automatic deletion which are documented in the [Meta Stack Exchange deletion faq](http://meta.stackexchange.com/a/5222).

Other than that, users with at least [10K reputation](http://mathoverflow.net/help/privileges/moderator-tools) can vote to delete closed questions, although you generally have to wait two days before you can do this. The question being very lowly scored will remove this restriction for [20K users](http://mathoverflow.net/help/privileges/trusted-user), so again downvotes help. Moderators, of course, can delete anything at anytime. In general, I think it is better to have more human input when culling questions, although it does require highly reputationed users to spend time on janitorial tasks.