you can dismiss heroes the same way as any other troops you've trained. Click on the portrait of the hero, click on the two litte arrows in the action list reading "More Actions", click "Disband". You can also disband single troops and heroes when you open up the "Economy" tab in your Ledger when you click the "Govern" button to see an overview of all your troops.
And I agree with parrottmath that a 1 gildar upkeep cost for heroes is very reasonable, I myself would expect my sovereign to pay me for my services roaming the lands or sitting idly on my ass in a city's pub to decrease unrest.