Answer: you definetly cannot stop ENSO. This is a global climatic phenomenon which is linked to the Walker cell and its influences on global winds patterns.If you wanted to stop ENSO, you should change the Earth's orbit, its distance with the Sun, its shape and composition, especially at its poles,.... As you see, forget about avoiding it.
Mitigation of ENSO feedbacks (rainfall/droughts) it is completely another point. And yes, put simply, of course you can mitigate droughts and floods.
In addition, ENSO has its own periodicity and it is not related to GHG emissions. It might be that the ENSO-related hazards are accentuated by global warming (due to GHG? I am not too sure, that is another story), BUT, I am not a climatologist and I haven't read anything about it.
I hope I helped you a bit (sorry this isn't much of an answer).
