QUOTE (Cheops @ Jan 26 2011, 11:14 AM)

Since you need the spirit's formula/true name I'd say that you kill it forever and ever.
THIS is the reason it exists. This is the reason its good.
If you have the spirit's formula, you can do so much crazy-batshit to a spirit.
You can force them to stop whatever they are doing, and come to you. At will. No drain. And you don't even have to be awakened to do it, much less have binding. (sm 108)
You can also bind free spirits at no drain. Yes, no drain. Specifically so. Its also worth noting that ANYONE can prepare the ritual to bind a spirit(arcana+logic, threshold 1), but then you need someone with binding to actually perform it. (like a mage-for-hire)
Since free spirits are automagically(literally) aware of all copies of their formula, and should be incredibly afraid of someone who has it.
Destroy Free Spirit isn't there to go 'lol, stunbolt is better'.
Its to go 'i'm tired of your shit. Come here, right now. Okay, now you're bound. Show me your home metaplane, then do nothing but twiddle your thumbs until your services run out. Sweet, we're here. BOOM, you're dead, permanently. Suck it.' or 'thats what i'm GOING to do to you if you mess with me again. BEHAVE.'
I'm fairly sure there are other interesting shenanigans to be done with a free spirits formula, but being able to compel and control them, and having that built into the spell itself is a big one.
Edit: Oh yeah. I'm fairly sure you can use astral tracking to find that free spirit anywhere, by using its formula. It would also make a great sympathetic link for ritual magic.
pbangarth: Thats pretty awesome, actually. I'd heartily endorse creating a Destroy Madam spell - pick up street magic and start looking for uses of a spirits true name/formula. There's plenty of other useful/annoying things you can do with it - just be careful it doesn't happen back to your spirit character!