It's magic... It doesn't cancel out that easily.. 
It's like the episode where levy and gajeel are fighting the two grimoire heart members and one of them uses their eastern solid script magic to make a loud screech but it doesn't cancel out even when he made another thing with solid script. Levy had to use her solid script to silence his in the end. Thus making the magic continue until either the user stops it or outside forces take handle of it.