[KoiYuki]

Oh boy.. It doesn't sound like you even watched the show, which would answer pretty much all your questions..

Spoiler for big plot spoilers:

Girls who make the contract have their soul separated from their bodies (which they rarely are aware of) in the form of a Soul Gem. It becomes tainted through magic use or despair. They can only get rid of the taint by killing witches and getting the Grief Seed the witch drops.

If they don't fight, they'll eventually have a tainted soul gem anyway (because just existing is using magic at this point, thanks to it not really being their body anymore, or due to falling into despair). And the real spoiler is that when their Soul Gem is tainted enough, they turn into witches.

Some of the girls have tried to kill Kyuubey too, but it's impossible, because it has countless bodies (and is a hivemind), so there's really no way out of it once you're in it (unless potentially if someone else wished you out?).

So basically if you don't fight, you're a witch (which is inevitable unless you get killed). You do fight, it'll only last so long until you're dead or a witch.