If Trump were to strike a plea bargain that included terms that he not run for public office again, what would actually stop him from continuing his presidency campaign?

If his campaign did officially cease operations, what would stop him from campaign like behaviors while claiming not to be running, but potentially being elected via write-ins?

What would stop him from being included on ballots anyway?

Agnew resigned as VP (1973), but I don't see specific details about his plea bargain. Even if the plea bargain did include terms not to run for public office, I still don't see what would have stopped him from running aside from public sentiment.

What would give such an agreement power? Who would enforce it?

Debs (1920) and LaRouche (1992) ran for president from prison, but that isn't really relevant to my question.

Rachel Maddow asks whether Trump should be offered a plea deal trading the threat of jail time for his agreeing to never hold public office again. Ron Liebman Barney Skolnik, and Russell “Baker, the assistant US attorneys in the Spiro Agnew case, answer as if such a deal is possible. 25:13

If Trump were to strike a bargain that includes terms that he not run for public office again, what would actually stop him from continuing his presidency campaign?

If his campaign did officially cease operations, what would stop him from campaign-like behaviors while claiming not to be running, but potentially being elected via write-ins?

What would stop him from being included on ballots anyway?

Agnew resigned as VP (1973), but I don't see specific details about his plea bargain. Even if the plea bargain did include terms not to run for public office, I still don't see what would have stopped him from running aside from public sentiment.

What would give such an agreement power? Who would enforce it?

Debs (1920) and LaRouche (1992) ran for president from prison. This doesn't answer my question, but informs what could happen without a plea deal and Trump is convicted.