You can check the specifics on Hertz's website under "Locations, Cars and More/Qualifactions and Requirements" but the simple answer is that, unless your specific rental contract listed a "No Show Fee" (usually only applicable for Prestige and specialty vehicles although I've never seen one) you can simply not show up for your rental resevation and it will expire.
Best thing to do however is to call and cancel. Again, unless your rental contract included a "Cancellation Fee" (I've never seen one of these either) there won't be one.
Obvious exceptions to this are pre-paid rentals through travel websites (Orbitz, Travelocity, Etc.) If that's your case, you'll need to deal with them.