How do I safely stop a long exposure early on a Canon camera?

Turn the camera off. The camera will save what has already been captured by the sensor and then shut down properly. You can then turn the camera back on. It's a much better way to do it than opening the battery or memory card doors.

Note that if you have Long Exposure Noise Reduction (LENR) set to 'On' or to 'Auto' under conditions that would cause the camera to take a dark frame, turning the camera off before the full duration of the set shutter time will result in no dark frame being taken. However, if the full duration of the shutter time has already transpired and the camera has begun taking the 'dark frame' when the power switch is moved to 'Off', the camera will continue with the dark frame for the full length of the first exposure and process the two frames together before recording the result to the memory card and then shutting down.