If there were a very successful online movement to prevent either major party candidate from winning, would the following scenario be possible? To express disdain for the current major party candidates, tens of millions of people write in "someone better" in their presidential election ballot (for the many states that allow write-in candidates). What would happen if "someone better" actually garnered the greatest percentage of the popular vote and neither major party candidate was even close to 270 electoral college votes?
A write-in candidate must register beforehand in all but a few states. In those few states where no registration of write-in candidates is required, some eligible person must show up afterwards and claim the votes.
When someone votes for an unregistered candidate, the vote is invalid and is counted as if it was never cast in the first place. So unless you find someone who is actually named "Someone Better" and fulfills all the criteria for being electable as POTUS and actually wants to be POTUS, the state is won by whatever candidate who is actually running got the most valid votes.
The stunt might get some publicity and might get you invited to some talk shows with the title "is this the end of democracy?", but constitutionally-speaking the situation is clear, so in the end you won't prevent one of the major candidates from moving into the white house.
neither major party candidate was even close to 270 electoral college votes?
If no candidate gets 270 electoral college votes, then the election goes to the House to pick among the top three candidates. See this question or this question for examples of discussion around this.
This is ignoring any difficulties around getting a nonexistent candidate on the ballot. Maybe we pick a dying but real person.
Oops. That won't work either. We have precedent on that. Horace Greeley died shortly after the 1872 election but before the electoral college results were certified by the House. The votes of the three electors who voted for him were disallowed.
The House has authority around certifying electoral college results, so simple assaults on the popular vote election won't work. If you want to effect change, you'll have to go the long way around. Start a movement. Find qualified candidates. Win elections. Or join some existing party and co-opt it the way that Donald Trump and Bernie Sanders did.