As I see it, there are only two options to stop Trump: (1) Prevent 1,237 votes at Convention, or (2) 3rd Party candidate OR candidates preventing either the DEM or GOP reaching 270 electoral votes thus the House selecting our next president. Both options remain viable however both have their risks, advantages, and disadvantages.
Convention: There are many ways to prevent Trump receiving 1,237 delegates on the 1st ballot such as changes to the rules allowing individual state bound delegates to vote according to their own beliefs, values, etc., or simply a certain number delegates abstaining on the 1st ballot. A course chaos would ensue at the convention and the "Branch Trumpidians" would be crying the rest of their lives.
3rd Party Candidate: I consider this option to be more dangerous with the desired outcome unknown until the night of the election. Let's say for example, Romney is on the presidential ballot in Utah. Six (6) electoral votes would be denied to Trump/Clinton. Let's say for example that Cruz is on the presidential ballot in Texas (i know the deadline is past however this can be challenged by the court system in TX) and therefore Cruz wins the 28 electoral votes. Another state where this could be accomplished would be Wisconsin.
Remember that Amendment XII to the Constitution contains the following:
...and if no person have such majority, then from the persons having the highest numbers not exceeding three on the list of those voted for as President, the House of Representatives shall choose immediately, by ballot, the President. But in choosing the President, the votes shall be taken by states, the representation from each state having one vote; a quorum for this purpose shall consist of a member or members from two-thirds of the states, and a majority of all the states shall be necessary to a choice....
The concern here is to make sure the preferred 3rd Party candidate is no less that 3rd place. I would assume the Libertarian Party is presently the greatest challenge to this 3rd place finish?
Choose your poison pill, either way it will get ugly! The fat lady has not even started to warm up yet!