Although the title might sound silly and so easy, there are lot of people having difficulties with this. At first turning Magento module can be easy as going trough System Configuration > Current Configuration Scope > Advanced > Advanced > Disable Module Output. However, be very careful, this action only disables module output as it says. If your module uses, let’s say some Observer functionality to hook into some part of the system and does some overriding then those actions won’t be disabled.
To fully disable module, you need to go to module config file, like /etc/ActiveCodeline_MyModule.xml, and set it’s active parametar to false, like: