No legal means of annulling the present administration comes to mind. I don't think one exists. Other possibilities include a revolt by the screaming memes of the left. That would likely result in a counter revolt by the screaming memes of the right. Then we'd have a civil war on our hands. (The left would lose because the right has more guns.)
So until the election of 2020, just hunker down, organize, strategize, campaign, fatten your coffers, and give priority above all else to finding a really suitable candidate rather than the scum sucking, bottom feeders who ran last time.