How to add only unrefered node of a content type in an entity reference field of another content type There are 2(two) Node types 


A
B


A has a node reference field to node type B.

If a node x of A type used a node y of B type as a node-reference then next time node z of A type will not use the node y of B type as a node-reference because x is already used as node-reference.
The Entityreference unique sounds what you're looking:


  This modules adds a checkbox to an entity reference instance to enable a check for unique multiple entity reference fields. With this option enabled it is not possible to add one and the same entity to a multiple entity reference field.