Using the current discussion regarding the Johnson Act as an example (unless there are more relevant examples):
Can the Executive branch instruct an administrative agency to not enforce or ignore a law that was enacted by Congress?
The Johnson amendment is part of the tax code, so to remove it would take an act of Congress. But President Trump can direct the IRS not to enforce that portion of the law, Herzig said. “Yes, technically, the only way to get rid of the Johnson Amendment is to have Congress repeal that portion of 501(c)3. It doesn’t mean the executive branch doesn’t have tremendous power in the ability to decide whether they’re going to enforce the Johnson amendment.” Trump said he’ll ‘totally destroy’ the Johnson Amendment. What is it and why should people care? - The Washington Post and AP Explains: The fuss over Trump and the Johnson Amendment
If a law is on the books, how can there be discretion in enforcing it?
Is there a difference in federal or state laws that are legislated?
Or in common law statutes?