Viruses are basically little shells with the code inside. They need to be able to inject their code into your cells to reproduce

If you break the shell, then you can get at the code which is fragile an inactivate/kill them. That's how heat, and certain chemicals work i.e. pure alcohol, because they break the structure of the virus

You can also have things that bind to the shell and stop them from being able to get into you cells (that's essentially how antibodies work) so other things (like enzymes) can break then down