Unless the rules are changed for this convention, Trump delegates are bound to vote for Trump in the first round and cannot break that commitment. If they vote otherwise, they are removed from the convention, and their votes are recorded as if they voted according to how they were bound.
[I’m not looking it up, but I believe that is Rule 16.]
If Trump has a first round win, it cannot be stopped. If Trump does not have a first round win, many delegates are still bound to him on a fixed number of subsequent ballots - the number of ballots determined by their state and enforced as described above by party rules at the convention.
My impression is that Trump is so far doing poorly on delegate loyalty and on recruiting unbound delegates. He is also doing poorly on attracting delegates to support him once they are unbound. Unbound delegates could be bribed by either side, as could bound delegates for votes after they are no longer bound. By tradition, those bribes are offered as patronage jobs. A candidate’s staff would not want to be caught offering cash to someone who rejected him and exposed the tactic.
The rules for the upcoming convention haven’t been written yet. The newly elected delagates from Trump, sCruz and Kasich will make up the bulk of the rules committee members, they will write the rules for this convention.