A little piece of paper called the Constitution. US Courts give a LOT of deference to challenges based on explicit constitutional powers. If a future DoJ decides to challenge trumps use of a pardon (don't forget, there are a limited number of parties that could have standing to challenge trumps use of pardons), courts would likely stay any prosecutions while the challenge is in motion. By default, the pardon would be valid until it was struck down. You can't prosecute someone who is technically not guilty by means of a pardon.
If trump does it before he leaves office, and disallows the doj from challenging it in court, there may not be a way of knocking it down.
But assuming he does it in secret and only lets it out he did it after he leaves office, another admin could bring the challenge forwards, but he would be technically not guilty until the pardon was vacated. He would be free to eat mcdonalds and tweet about everything and anything. He'd probably even get tv time whenever he wanted it.
Sucks, but checks and balances are taken seriously.