Bukkit

Hi, i would like to know if I can prevent a mob from galactcraft to spawn

Unfortunately, not in the current version. Possibly in the next version (which should be out soon™), but this plugin can only control mob spawns that go through the Bukkit System. It is possible that Galacticraft bypasses this system, and so would mean that the answer to your question is no. I'll look into to for you.

pleeeease, do this! The Skeletal Boss is crashing my server.. This is so sad..

Could you maybe give me an idea of how you have Galacticraft installed? Are you using the BukkitForge plugin to install the mod, or are you installing Galacticraft directly into Bukkit? Don't worry if you don't know, it will just help me work out if I can fix this for you.