Limit listings per user account for admins and directory members

I would like to be able to limit the number of listings blog admins and directory members can create.
This can be a global limit that applies to all admins and directory members.
Some background:
I do not charge for directory listings - any admin or directory member on the multisite network can create listings for no charge.
Directory is installed as a blog on a multisite network
The plugin is activated just for the single blog site.

I would be fine with a hack that limits everyone to a single listing.

Thanks