Based on the rulebook, the following appears to be true:
1) When you join an alliance, you receive the Alliance Marker for that alliance.
2) A player may only belong to one alliance.
3) Even when you leave an alliance, you keep the Alliance Marker. You simply flip it to the betrayer side.
Based on this, it seems pretty clear that there is no way to remove an alliance marker once you've accepted one. In the situation you've described, where everyone has left the alliance but one player, that player remains in the alliance by themselves. They still benefit from the +2VP on the tile, and no longer need to split their score with another player. Also, the player who left the alliance may never join another alliance, because they still technically have an Alliance Marker. They also cannot change their mind and return to the alliance at a later point in the game.
If you're in a game of 6+ players, you could invite another player to join in your alliance, assuming that they do not have an Alliance Marker from another alliance already. In a 4-5 player game, you're stuck in an alliance of one...but at least you get some extra points.
So choosing your alliances should be done carefully, as the choices you make now will impact the remainder of the game.
[Edit: Typo]
1) When you join an alliance, you receive the Alliance Marker for that alliance.
2) A player may only belong to one alliance.
3) Even when you leave an alliance, you keep the Alliance Marker. You simply flip it to the betrayer side.
Based on this, it seems pretty clear that there is no way to remove an alliance marker once you've accepted one. In the situation you've described, where everyone has left the alliance but one player, that player remains in the alliance by themselves. They still benefit from the +2VP on the tile, and no longer need to split their score with another player. Also, the player who left the alliance may never join another alliance, because they still technically have an Alliance Marker. They also cannot change their mind and return to the alliance at a later point in the game.
If you're in a game of 6+ players, you could invite another player to join in your alliance, assuming that they do not have an Alliance Marker from another alliance already. In a 4-5 player game, you're stuck in an alliance of one...but at least you get some extra points.
So choosing your alliances should be done carefully, as the choices you make now will impact the remainder of the game.
[Edit: Typo]