At Admin Panel -> Settings -> Switches -> "Verify reciprocal link on submit", you can choose to have all 'reciprocal' type submissions be checked for a valid reciprocal link at time of submission. If the link is not there, the submission will be rejected (the submitter will be shown instructions from your 'suggestlink_cantfindrecip' language item).

If you want all submissions to be reciprocal, enable the "Only allow reciprocal and sponsored" switch as well.