Go online in self serve, you can switch your plan to a "Seasonal Hold" This blocks all network use (SMS/Calling/Data) of the phone for the duration of the seasonal hold. The cost is 15$ a month and just keeps your number active. You can have the seasonal hold for up to 6 months.
Quick note about seasonal holds: If you are on a grandfathered plan (A old plan no longer available) then you will lose out of it and be forced to get a plan that Koodo offers at the time you want to remove it from a seasonal hold.
If you'd like to suspend your service but keep your current phone number and your grandfathered plan, a better option is to cancel your service entirely (so you pay absolutely nothing while you're away) and then reactivate it within 90 days.
Beyond 90 days, your phone number may be given to someone else, and the grandfathered plan will no longer be available. The 90 days is essentially a grace period within which you can resume your Koodo service as you left it.