...a new crystal form of aspirin was claimed, after years of having stable polymorphs.
Well, they still discover new forms of (water) ice, despite ice has been known for quite a few years. The new ones appear under extreme pressure, cheating, OK.
The answer against polymorphs must be to impose how the melt or solution is to crystallize, and this is what Czochralski improves, as is tunes to melting or dissolving conditions (temperature, concentration) through all the liquid except the small zone where the desired crystal form is already present.
It's, as I believe to understand it, what lets grow single crystals of silicon, because seeds don't form elsewhere as conditions are melting instead of freezing. The same reason must prevent seeds of organic "heteromorphs" to appear, and the liquid or solute must aggregate only to the existing solid as the conditions permit it only there, and then follow the available crystal form.