Denmark should reply by saying "Sure, just become our colony of Vinland again and adhere to our rules, laws and customs and you can get access to Greenland"

Honestly, a three word reply to Landry would suffice: persona non grata.

Pretty sure anybody from Trump's cronies would ignore it, so I doubt that would help. The above would actually bring them down a peg or two and show them that they can't get everything they want by just commanding things.

Also, it would totally ridicule Trump, which would not go well with his base at all. That's the way to deal with autocrats, not with logic (as they and their followers will brush it aside and create their own reality), but by ridiculing them in front of everybody, as they can't stand it and their followers are furious for letting it happen while it emboldens their enemies.