You don't need to put the whole block instance and remove it.
However, you do need to disconnect the new coordinate where you are trying to put the block from your pathfinding system to see if the starting point is connected to the end point.
That means, you need to check the state of the pathfinding map where the new coordinate should be perceived as not passable. Then see if there's any available path. If no, do not permit the action of putting the block. If yes, execute the action.
By saying pathfinding map, I mean anything that represent the world of available paths (only care about paths), it's not neccesarily a map, depending on how you do the pathfinding, like Navigation2D, it can be different, but the idea is the same.
Overall, the idea is that when pathfinding, we only care if the coordinates of the world are connected (passable) without considering anything else (Monsters, combat, etc.), it should be separated from the game world. Then, before placing the block, we view where you just clicked on as a blocked path. After the action (Either executed or not), if you are keep tracking of the world available paths, simply updating it according to what action has been executed.