Those snowbirds either should voluntarily stay the fuck away for the foreseeable future, or their countrypeople should impress upon them that sentiment by whatever means they see fit.
This is already affecting people I know. People who come for the winter are selling houses and changing plans. It just impacts the US tourism economy. Needless, self-inflicted wound on America because of one man.
This is probably "newly" enforced but if they fly they should automatically receive an I-94 at immigration and I'd they enter by car they need to specify to the agent that that are staying more than 30 days or travelling more than ~50 miles from the border (I'm not sure about the distance) and apply for an I-94 at the border crossing, it's the same rule for Mexicans, the difference is that the South border has checkpoints where you can't cross with a visa without an I-94
Many who fly would have already received the required I-94 form, and they don't have to register if they've previously submitted it. Most driving to the U.S. aren't handed an I-94