So it is possible to disable "propose alliance", right?
I think that's the best way to get "only one player can get victory". It disables all of the more OP allied stuff (gifting), and it certainly disables victory for multi-empire alliances.
Peace could also be disabled, but I think that restricts too much gameplay.
With or without peace allowed, players could get into agreements (I don't attack you, you don't attack me, I take this and that planet, you take those, you can focus on conquering John Doe, I will focus on Jane Doe, etc.). But the fact that supply won't be propagated between supply groups if differents empires entangle their spaces will discourage close cooperation: each one will prefer to have their planets on their space (except for research-focused colonies), and to fight their own battles because no repairs and no fuel when you are at your partner's space sucks balls.
An alternative way could be to allow alliances but dissallow victory for alliances of more than 1 player (if that's been implemented). Thus, people could play alliances during the early stages of the game but always keeping in mind that your current ally will necessarily be your foe later on. This would make concessions to your ally more risky for you, forcing you to ponder if helping him out now (supposedly benefical for your immediate goals) will take a toll on you later on if your ally grows stronger than you in the process. Therefore, the diplomatic decisions would be harder (instead of an undoubtable win-win when allied victory is allowed). Going for a solo victory from start could be an option, but (very likely) those that choose to enter alliances will be able to grow faster.
So, if what we want is to promote the solo-playing at it's full potential and not give unnecessary advantages to diplimatic players, the first option (disable "propose alliance") is the best one IMO.
If we don't mind temporal alliances early-mid game (which could force all players to enter into alliances for survival, including L29Ah ), the second option (victory allowed to alliances of maximum one player) could be funnier.