A slow week at work and I'm posting far more than normal. Oh well.
Paris Sophia's comment about driving things away in Ars Magica made me think about summoning and banishing things - how would you do it?
When you summon a spirit, you pull it from wherever it was to where you want it to be - logically you might use the same guideline for sending it away but since you can't sense where it was, you can't, by standard Hermetic magic, send it back. You can always use the DEO equivalent to destroy them, but I would have thought most Magi would be fairly leery of permanently weakening the Magical Realm like that, and depriving themselves ofa future servant into the bargain.
Is there anywhere in the rules which addresses where spirits are when summoned, and where they depart to, if indeed they do so? Do spirits have the ability to return of their own free will?
Banishing would work as a Rego effect compelling the spirit to "return promptly and peacefully from whence you came". It will be easier than summoning (a.k.a "Come here at once") because it isn't cast at Arcane Connection range.
It all depends. Yes, no, some do, some don't, sure why not? Spirits, like fae or demons, are not something that should be given a single formulaic definition and all built to fit. The traditional stories about spirits vary, and the plot twists that make a good Saga vary even more.
I'd recommend that, as a SG, you don't even both writing it up - start with a solid idea of what the challenge/encounter is, and then adapt the details to the play and the story as you go. Make a good, scary, significant story - and have fun with it. That's the goal, right?
Note that with hermetic summoning and banishing spells, the spirit/creature/thingy summoned comes to the caster by its own devices. See the "Summon Animals" supernatural ability in HoH:S for comparison: it explicitly states that the animals come faster than they should be.
How then is the summoning guideline any different from the commanding guideline? That interpretation boils down to "Come here now" vs "Come here now" with less penetration. I've always interpreted Summoning as, effectively, the spirit equivalent of teleportation, pulling the target from where it is and bringing it to you.
Summon Animals is, amongst other things, nonHermetic because you don't need to be able to sense the animals to affect them.
Up to a point. One of the joys of Ars Magica, for me, is that the universe is defined in terms of the underlying properties and that within that framework the stories are self-consistent. It all makes sense. Since immaterial things exist but are not necessarily present, and can be banished, they must come from somewhere. Having it be arbitrary and based on whim doesn't, for me, generate a good story, it generates random plot-twists which don't make sense in context. Besides, the traditional stories about spirits are just that, stories. There's no reason that they all have to be true in Mythic Europe. You can bet that several Hermetics will have invested a lot of time in working out where spirits come from and go though, even if they haven't managed to find a good answer.