Most of the big money goes to the States, but if President Trump halts any money on day one, he will have their attention.
Then the monkey is on their backs to get the policy changed (could take years). Meanwhile, he could sick the Justice Department on them with investigations and prosecutions.
And then case by case, he could tweet out how screwed up the mayors are, using their taxpayers’ money to keep individual “poster boy” violent criminals loose on the streets to prey on those same taxpayers.
There is a lot that could be done, that would be great politics, as well as effective in changing policy. You can always find another good cause to spend whatever money the sanctuary cities forfeit.
There is also the “slow pay” method to already congressionaly allocated funds as well as a refusal to consider any federal grant applications for any entity in the entire state of California. He is never going to win CA in 2020 by doing what every other RINO has done since Reagan.