The dilbert cartoon is spot on, fungibility is an issue if we only play a shell game. But if we stop imports of oil from Saudi, Iran, Venezuela, then even an artificially constrained supply will feel the glut on the market.
The only way to cause a glut is for consumption to decrease. (In reality, most of the middle east producers can decrease production very easily to keep the supply/demand intersection where they choose.) If we were somehow to say that we will not use barrels of oil that happened to be pumped from the ground in one or all of those countries, does that mean we stop consuming that same number of barrels to cause a "glut", (which wouldn't happen anyway) or do we replace them with an equal number of barrels from other countries freeing the "bad oil" for use by others?