Do UN delegations have a certain limit in size? The reason given for denying the visas seems strange and unilateral. Even a UN resolution should not impact the choice a country makes regarding its representatives.
Also, I consider unproductive these type of tactics or the ones related in the article. UN is a place for all countries to sit and talk. Including dictatorships and fundamentalist regimes. If the host nation is uncomfortable with that idea, move UN somewhere else.
Probably there is something that is not related in the article. For instance, I am curious if US can deny the leader of a county access to UN or to a country's UN ambassadors (Ahmadinejad will still attend, the ones denied are just ministers - same nuclear Iran problem).
P.S. Even when US was bombing Afghanistan, the taliban Afghan delegation was still present at UN, if I remember correctly.