We can't stop a planetary take over, since we can't be everywhere on a planet at the same time. It's statistically impossible, even pushing the limits of my capabilities.
And...uhh...I seem to remember in one instance, he's pulled a moon out of the sky, busted it into pieces and dropped it down on someone. I don't think you can get big enough to stop that if he did it to you.
That doesn't mean we're helpless, but it will take more than trying to take on the forces that Thanos has at his disposal. Head on, I mean.
There is not going to be an easy fix, any time soon. It will take working in conjunction, on several fronts.