Donuel, I believe it was Canute. But he did it not to show off, but to demonstrate that he had no powers over the elements, and that only God could control the sea. He's often misrepresented in that story.

Couldn't they just build a wall around Trump himself?

Has anyone seen the giant 'Trump Baby' inflatable balloon which people are saying will float overhead in London during his visit, as a protest?
I think it's a spoof, but they're saying permission has been granted.
Do you suppose it's true?