Can a abdomen:pelvis ct definitely rule out a bowel blockage?

Yes: ACR specifies: “Oral contrast should not be used if high-grade SBO is known or suspected. Oral contrast will not reach the site of obstruction, wastes time, adds expense, can induce further patient discomfort, will not add to diagnosis. A small bowel diameter of 2.5cm achieves the highest combination of sensitivity and specificity for the diagnosis of obstruction.