How do I delete a voice mail before listening to it to avoid using up my prepaid minutes? And if it's not possible, is it possible to turn off my voice mail so it does not automatically kick in when I don't pick up?
Afraid nor without calling your voicemail from another line. And you cannot fully disable your voicemail.
"From a landline or another mobile phone, dial your area code plus your phone number, press the * key, enter your password, and follow the prompts. "
You can place an extended suspension message though for people. It's a specific type of message via options in voicemail.