One of the concepts underpinning the UN is that it serves as a forum wherein rivals can communicate and thereby avoid more serious conflicts.
The US, as host country for the UN, typically does not interfere in the selection of delegates sent to serve there. Obama has defied decades of protocol to deny the entry into the US of Iranian ambassador to the UN Hamid Abutalebi.
The rationale for this idiocy is that Abutalebi was involved in the 1979 Iranian hostage crisis. Insofar as that is true, it makes him guilty of nothing other than being an Iranian patriot. Should we be surprised that countries send people to the UN who they consider patriots?
This kind of meddling proves the point for those critics who claim that the UN is just another front for "The Empire."