I understand that spectrum is considered a national resource for many governments. It is akin to land, water and other resources. It is also a scarce resource. I wonder how is it that government controls unauthorized agencies from using this spectrum. Like water is held behind dams, is there any way to prevent agencies from simply accessing parts of the spectrum? Please provide some intuition too.
They control the spectrum with regulations:
Below is the diagram of frequency allocations in the united states. When people think of radio, they typically think of only the FM or AM parts of the spectrum. However, any frequency of electromagnetic radiation can be used to communicate with others.
There is nothing that a government can do that could prevent someone with sufficient technical knowledge from using any frequency band. Therefore, this is probably more of a question about law than physics.
Nevertheless, to answer the original question, there is no way to barricade a portion of the spectrum and still make it useful. Certainly, jamming would be able to prevent use of a certain area of the spectrum, but that would also defeat the purpose of trying to keep unauthorized traffic off the band.
A HAM radio operator (or anyone else, for that matter) could potentially jam the cell phone bands which you mention in the comments, however doing so is illegal and may carry a heavy fine if the government is capable of proving that you were intentionally interfering with the band.
(TL;DR) As far as providing intuition on how this works: you can build an electronic oscillator that is resonant for any frequency. If you amplify this signal sufficiently and connect the output of the amplifier to an antenna, you can transmit on any band.