When the USA greed to have the UN in New York , it accepted the fact that it has to issue Visas to UN members representatives. You say the Iranian is a terrorist. Many in Iran think the American Embassy in Iran was engaged in terrorism and subversion of the Iranian state/People. The proof is very well documented in history if you care to research it. If the USA does not want this role , they should not have hosted the seat of the UN in New York. Now if you do not want this to happen , have the USA withdraw from the UN and move the UN headquarters to a neutral country. Fine by me.
Ofir Akunis officially replaces Danny Danon as minister of science, technology, and space (Haaretz)
from the article: Iranian hostage-taker 'not viable' for UN appointment, White House says