code: public void markAsUnspent() {
          if (parent != null)
              if (log.isDebugEnabled()) log.debug("Un-marked {}:{} as spent by {}", getParentTransactionHash(), getIndex(), spentBy);
          else
              if (log.isDebugEnabled()) log.debug("Un-marked floating output as spent by {}", spentBy);
          availableForSpending = true;
          spentBy = null;
      }
docs: Resets the spent pointer / availableForSpending flag to null.