What you *can* do is effectively disable the heater by:
* Closing the heater control valve: This is the most common and safest method. Locating and closing this valve (it's usually a cable-operated valve under the dash or near the firewall) prevents coolant from flowing through the heater core. The coolant will then circulate through the bypass built into the engine's cooling system.
* Disconnecting the heater hoses (NOT recommended): This is a *very bad idea* unless you immediately plug the open hoses to prevent coolant leaks. Even then, it's a temporary solution at best and risks damaging the engine. It also means no heat in the winter, obviously.
In short: Don't try to completely bypass the heater core. If your heater core is leaking, repair or replace it. If you just want to turn off the heater, close the heater control valve. Attempting a full bypass is risky and will likely cause more problems than it solves.
