Nope, theres nothing to stop them from messaging you, unless you block every single player in game.
In reality though, if you want to play this game and not get raided, you're gonna need to find one of those alliances.
My advice would be to ignore the ones that spam you messages and offer startup aid, and go look on the CN wiki for an alliance you think you will like. Most alliances will give out aid anyways, if they have to bribe you, ignore them.