in the konqueror file menu, navigate to the Settings > Configure Konqueror menu item.
on the left frame, look for History Sidebar, click it's icon, and click the Clear History button in the main window.
on the left frame, look for Cache, click it's icon, and then click the Clear Cache button in the main window.
on the left frame, look for Cookies, click it's icon. The main window has 2 tabs. both tabs have a Delete All button. click them.
click the Apply button and then the OK button. all done.
to clear the history in your location address bar (where you type urls in konqueror), make sure there's some text in the location address bar and right click inside the location address bar and choose Empty Contents.
this is for konqueror 3.1.1 on kde 3.1.1 but hopefully it's the same thing for your version of konqueror and kde.