Is it possible to checkmate someone with just a knight?

No. A Knight with only a king cannot checkmate the other King, even if that King has no other pieces. Under the official rules of Chess when a game comes down to one player having only a King and the other player having only a King and a Knight, the game is automatically a draw. The same goes for a King and a Bishop against a lone King. This is why the Knight and Bishop are referred to as "minor pieces" while the Queen and Rook are "major pieces." A King and Queen or a King and Rook are able to checkmate a lone King.