Node reference - limit so node can only be referenced once I have two nodes 'Runner' and 'Buddy'. I'm using node reference to create a select list of buddies on the runner node. 

The Idea is that every runner only has one unique buddy and two runners don't end up with the same buddy. 

When editing the runner the buddy select list shows all buddies, I would like it only to show buddies that don't have a runner. 

How would I achieve this?
Node reference (for Drupal 6) and Entity Reference (for Drupal 7) fields can be configured to use a View to show the available choices.

So, first create a View that shows all buddies that are not related to a runner. Then configure the field that points to the buddy to use that View. 

To construct that View you may use the Corresponding Node References. It syncs the node reference between two node types which have a nodereference to each other. WIth this module your View only has to list buddies that have no runner related. This module is for Durpal 6 (I'm guessing your are using Drupal 6), but there's a similar one for D7.