Answer:
No form of niacin helps to clean out THC. Flush-free niacin is doubly bad, because it has no biological activity whatsoever. Flush-free niacin is "inositol hexaniacinate," which is an "inositol backbone" with six niacin molecules attached to it. The body can't use this.
Now...niacin doesn't clean THC out of your body, but it will lower your cholesterol, which is good too. Flush-free niacin won't clean cholesterol out of your bloodstream OR pull THC out of your fat cells.