Apparently threaten to cut off funding and/or sue the state. Think that happened with... Indiana a few weeks back. DOJ sent the Governor a letter saying that the state's new nullification law was essentially a waste of paper, and that trying to imprison or impede Fed agents trying to enforce Fed laws was a crime.
I've read that as well, but I wonder if there is an argument against the Supremacy Clause when the federal government laws are in opposition to the Constitution.
Now, that I look I see that it has already be argued to no avail.
Liberals love it when states void their pot laws, and ignore the federal drug laws. Hell even the media says it's legal to use marijuana in some states, they just fail to mention that it's still a federal crime and thus not legal.
A few states try to do that same thing with gun laws, and you see their progressive minds wake up, panic and get all upset.
It's an interesting concept, and at face value (to me, at least) seems like it'd hold up. Unconstitutional laws (laws that are an inherent over-reach of government power) are supposed to be illegal outright. Good luck convincing SCOTUS or Joe Voter though.
ETA: Acera, don't make me laugh man. Selective enforcement = you aren't doing your damn job right.