The short answer is probably not. The Republican Party’s most important rule for determining a nominee is that the candidate must have earned the support of 50% of the party’s delegates +1. For the 2016 cycle, that magic number is 1,237 delegates.
Since the Indiana Primary on April 26, Donald Trump has been the only Republican candidate for whom it is mathematically possible to earn that many delegates – on top of that he is also the only Republican candidate still in the race.
However, political parties are not mentioned in the Constitution– which complicates things. While they aren’t technically private institutions, they aren’t public ones either. That means that theoretically a party could deny nominating a candidate if they choose, but this would be unprecedented.
Furthermore, Republican leaders – including RNC Chair Reince Priebus – have shown no willingness to “steal” the nomination from Trump, and have actually called for the GOP to unite behind him.