Starve the beast. Raise their rent.
There is no rent. The land was donated to the UN by the Rockefellers. Funding for the buildings came from an interest free loan via the US Congress. The land is considered extraterritorial to the United States. There are UN offices in Geneva, The Hague, Vienna and Nairobi. This section on relocation from wiki is interesting since this has been a long standing wish by Freepers.
Relocation proposals
Due to the significance of the organization, proposals to relocate its headquarters have occasionally been made. Complainants about its current location include diplomats who find it difficult to obtain visas from the United States[125] and local residents complaining of inconveniences whenever the surrounding roads are closed due to visiting dignitaries, as well as the high costs to the city.[126] A US telephone survey in 2001 found that 67% of respondents favored moving the United Nations headquarters out of the country.[127] Countries critical of the USA, such as Iran and Russia, are especially vocal in questioning the current location of the United Nations, arguing that the United States government could manipulate the work of the General Assembly through selective access to politicians from other countries, with the aim of having an advantage over rival countries.[128][129][130] In the wake of the Snowden global surveillance disclosures, the subject of the relocation of the UN headquarters was again discussed, this time for security reasons.[131]
Among the cities that have been proposed to house the headquarters of the United Nations are Saint Petersburg,[132] Montreal,[133] Dubai,[134][135] Jerusalem,[136] and Nairobi.[126]
Critics of the relocation say that the idea would be expensive and would also involve the withdrawal of the United States from the organization, and with it much of the agency’s funding. They also state that the proposals have never gone from being mere declarations.