Can presidential pardons be repealed?
There is no clear process to repeal a presidential pardon. Article II, Section 2 of the Constitution gives the head of the executive branch "virtually unfettered discretion" to "grant Reprieves and Pardons for Offenses against the United States," including pardons, commutations (substituting a less severe penalty for a court-imposed punishment) and reprieves (delay of a sentence's execution) for federal criminal offenses.
Presidential pardons aren't subject to judicial review. They are plenary in nature, meaning beyond legislative control.
The power to pardon is not entirely unchecked. Presidents may not grant clemency if they have been impeached or are in impeachment proceedings. A 1974 federal district court case ruled that clemency must be "directly related to the public interest" and not unduly infringe the recipient's "constitutional freedoms" with any conditions imposed.