You can follow these instructions to disable your voicemail: 

1. Call your voicemail by holding the number 1 on your dial pad.
2. Select option 3 to “Manage your greetings”.
3. Select option 4 to “Manage your Extended Absence Greeting”.
4. Select option 5 to block callers from leaving a message after hearing the Extended Absence Greeting.

I'm not entirely sure if that works for prepaid, but it's worth a try Edit: just tried this and it works... So yeah definitely am option!

Alternatively, you can switch to a third party service like Youmail (free for light usage) It has really nice call blocking features plus a robust spam filter. If MilkyWay's solution doesn't work for you, it's worth a try! (Edit: it would work)

Also, make sure you register your number on the do not call list: https://lnnte-dncl.gc.ca/en

Thank you for all your replies.  It seems to have worked.