To clear measures, first select the measures that you want to clear. Then right-click and choose the Clear Selected Measures option. This deletes all the notes from the selected measures, but leaves the now empty measures themselves in place. You can also just press the Delete key instead of using the menu.
Alternatively, you might want to completely remove some measures from your score. Select the measures as before, then choose the Delete Selected Measures option from the right-click menu, or use the Ctrl+Delete keyboard shortcut.