Chris,
The "Helms proscription" is to be found in the Omnibus Consolidated Appropriations Act for 1997 (Public Law 104-208)-- the "Helms-Dole" Act-- passed September 30, 1996: section 581A, pp. 570-1.
The language of the proscription is not as strong as I had originally suggested. It bans /voluntary/ UN contributions if the UN /enacts/ (not "studies") any sort of international taxation. Apparently Helms had called for tougher language but was forced into a compromise.
I have the actual text somewhere. If I find it, I'll post it.
Best,
Hank Sims