They could, you know, just stop voting for him in these contests. That would help.
SC decides the GOP nomination, not IA or NH.
Not exactly true. SC went for Reagan in 76. Technically New York has been more accurate than SC, picking the winner since 1972. I am sure there are other states as well that go back further than 1980, the year SC began its streak.
I think Florida also has a streak of picking the winner going back to 1976. In any case, correlation is not causation. South Carolina always goes for the guys who's already the national frontrunner at the time of the primary, and the GOP always nominates the early frontrunner. Doesn't necessarily mean that the one causes the other.