What are you talking about? Taxing who? The UN already mandates (as in "Come on, please. Pleeeease. We'd really appreciate it.") that certain members pay certain amounts (those amounts are determined by weighing factors like the national incomes of each country, etc.) Who do you think pays for the UN? Answer: The US. We are by far the largest contributing member, we pick up the lion's share of the UN tab. We use our heavy financing to attempt to force the UN to comply with decisions that favor American interests. Sometimes it works - sometimes it doesn't.

The UN has neither the right nor the power to "Tax" the individual citizenries of its member States (though, in a backwards way it already does. For example - the US government gives hundreds of millions of dollars to the UN and then the US turns to its citizens to recoup the expense via federal taxes,) and even if it did not one member State would comply. As I said before, the UN already draws its funding from its membership (in practice, though, it's not so much the entire membership that chips in enough money to keep the ship sailing as it is the US that forks over the big bucks.)