You can use ovi maps offline as long as the maps are there in your phone memory or memory card. Map is stored in a folder usually named as 'cities'. Online mode or an active internet connection is required to present extra information like traffic updates etc. If you like to use maps in offline mode only, then you can turn off online mode through, open maps-> click on settings (a pic of wrench)-> internet -> connection - offline. Please note that, in offline mode extra features offered by nokia maps like landmarks etc won't function properly. Also in-map apps like trip advisor also won't work.
If you only want to turn off accessing maps via wifi, then go to settings- application settings- positioning- positioning methods- un check wifi network. Now maps will never access wifi network for positioning. Enjoy!