There is a $2 add on that the rep at the store can add to block all data charges. The only thing you'll be able to do with your data turned on is send picture messages, and you will not be charged for those. Your phone itself will still count that as data being used, but Koodo will not charge you for that data.
EDIT - just because you have no data included in your plan, you still have Pay Per Use data, and that is where the overage comes from. If you do not want to use PPU data, get the $2 Data Block Add-on