Yes, you can. To delete an offline map, go to Menu > Offline locations. At the top, you will see an overview of the countries you have downloaded. Click on the button next to the country you want to delete, and all maps and data for that country will be deleted. The country will then no longer appear under Downloaded countries, but under Available countries.
You can also pause (cancel) a download. The app will then stop downloading the file and everything you had already partially downloaded will disappear from your device.