Here is a logical proof: Gods existence is unlimited and He encompasses all things. There cannot be other gods besides Him, because He encompasses everything and His existence is unlimited so there is no room or space for another god to occupy. An example is, if you have a fountain of water that is full all of the way to the brim and this water occupied all of the space available in the fountain, how could you add more water to the fountain? It would be impossible, because there is no space to add anything. God occupies all of existence and there is no room for another god, because God is already encompassing the totality of existence.