The police, including Sheriff Arpaio, do not have the authority to randomly stop people they think "look Hispanic", ask them for their ID, and then detain them for no apparent reason.
We do NOT have a "your papers, please" society. At least, not yet.
Police need a probable cause to stop someone. "Looking Hispanic" is not probable cause. "Driving while black" is not probable cause.
Sheriff Arpaio violated a court order because he felt like it.
A president's power to pardon is absolute, but Trump would not be not pardoning Arpaio over a concern about due process but solely to please his anti-Hispanic base.