They can't stop Trump from getting the nomination if he heads to the convention with more than the required 1200+ pledged delegates needed to secure the nomination. By law, on the first ballot, the delegates MUST cast their vote for whom they are pledged.
If no one has the 1200+ pledged delegates needed, then after the first ballot voting is completed where no one has secured enough votes for the nomination, then the delegates are released from their pledges and free to vote for who ever they wish on the second or subsequent ballots until a candidate has secured enough votes for the nomination.
Must have been that Romney endorsement