If you drag over Ableton Live effects on a running system, by default the effect is enabled. This is not fun, as you suddenly have a big flanger effect running on let’s say the master channel. However, I noticed that if you keep the command (apple) key down while dragging over effects from the Live devices browser, the effect is then disabled by default.
It’s usually good to try out various UI elements in the application with the option, control, command or shift keys, or even combinations, to see what happens. Sometimes you even get visual cursor changes indicating that the behavior is different…