How can I prevent snow from sticking around in Minecraft?

Since you said you're the admin, and you don't mind using hacks, you could move the spawn point. In a similar vein, you could set the biome to be something different. For vanilla, the easiest way I know to accomplish these tasks is to use an editor such as MCEdit. Both tasks are relatively straight forward, however I don't know the steps required off hand.

Having said that, it's hard to tell if MCEdit (in particular) will work with your world. It appears that it's hit or miss with some mods, and even different versions of the same mod, especially if those mods introduce naturally generating blocks (or if your world already contains non-vanilla blocks). Back up your world before attempting.

If you like the biome the way it is, but you just want to prevent snow, get the plugin World Edit. There is a feature that allows you to turn off weather in a certain area, and therefore prevent snow. If this feature doesn't exist anymore (which is possible considering I haven't owned a server or been admin for over a year) there is a simple way of making a canopy.