Warning: This is an advanced topic and you should probably consult WSN support about it first if you're not a programmer.

At Admin Panel -> Listings -> Listing Settings, there's an option "Only Show Listings Where." This is an advanced option intended for use in rare cases by people somewhat familiar with MySQL.

In this field you can supply any MySQL condition, and it will be used inside the "where" clause of the queries which select links. Thus for example if you put recipurl != ''
in the field, it would effectively prevent any links which lack a reciprocal URL from being shown on your site.

This sort of method can on occasion be useful when you're doing multiple installs and want to share most of the content. You could set up a custom field to mark which installations a listing should be displayed on, and then use the filtering condition whichsite='applicable' OR whichsite='' to filter.