Geoffrey,
Currently, automatic deletion of older scans does not exist. This is a great suggestion, and I have added to the list of possible features to be added to our next major release.
As far as short term solutions, besides manually deleting old scans, there are not a lot of options. We could remove some of the heavy data (raw finding) associated with older closed vulnerabilities using a MySQL query. Using a query is not a pretty solution, but it will cut down on the footprint of older vulnerabilities if you are hurting for space.
Daniel Maldonado