I don't know what, short of impeachment and removal, any of us can do to stop it.
I know I'm reaching here, but maybe someone with standing (who?) could file suit to have the courts intervene, maybe under the theory that this is an end-run around Congress's power of the purse, and that this kind of settlement paying taxpayer money out of the Judgment Fund to the president's allies is best seen as an unconstitutional bypass of congressional appropriations?