When you add key bindings to your user settings, you're doing just that; adding a binding. Binding a command to a key does not automatically remove it's binding from the original key.
If you don't want a key combination to do anything, you need to create an explicit binding on it to tell Sublime you want that key to do something else. If you use a command that doesn't exist, like
noop, then the key binding will stop doing anything.