The "Utah Sovereignty Act," which went into effect this year, is what they claim allows them to do this.
The Legislature may, by concurrent resolution, prohibit a government officer from enforcing or assisting in the enforcement of a federal directive within the state if the Legislature determines the federal directive violates the principles of state sovereignty
Which... why even have a U.S. at that point? Just secede if you feel that way.
The land doesn't even belong to them. Between the Bureau of Land Management, National Forests, Parks, National Monuments, National Recreation and Wilderness Areas the federal government owns over 70% of Utah.