There are two answers to the question of “Why can’t Donald Trump be stopped?” One is that some people don’t actually want to stop him. The other is that any of the obvious solutions have more negative consequences than leaving him in. In both cases, Trump is not the problem, only a symptom.
Donald Trump is polling an average of 39% across the primaries tomorrow. If he under-performs the polls, (which is common) he’ll take about 35-36% of the popular vote. In a 4 man race, this could give him a clean sweep. Trump will likely pull away from Cruz in the delegate count. Besides getting all the delegates in Florida and Ohio, he would get more delegates than Cruz in the other three contests. It would also make Kasich a total loser and Rubio unable to win his home state (although Trump has a bigger, more fantastic home in Florida).
What we have now is a race where Donald Trump is the front runner and Ted Cruz has no reason to get out because he is a solid competitor to Trump. If Kasich loses Ohio, he would need to collect 144 delegates in the other states and 100% of the delegates for the rest of the primary season. Rubio would need 100% of the other delegates and 44 delegates tomorrow. It is already mathematically impossible for either one to get the nomination.
If this were about stopping Trump, the GOP and the donors would tell Rubio and Kasich to get out of the race tomorrow so Cruz can fight Trump. With a 60%+ anti-Trump popular vote, Cruz would have a chance of keeping Trump from winning half the delegates. The truth is that Cruz is probably less desirable than Trump. The Republican establishment can live with Trump losing, which is a good bet if Democrats can spend a billion dollars and some free media destroying him. Even if Trump somehow wins, the GOP amasses a nice war chest and lets him rage while planning their post-Trump return.