The weaknesses are the natural fracture points in a demon's soul. You can't "generically" break someone; they're going to break at a point of weakness. Summoning demons with a specific weakness is just a good way to know both how to break them and what that means.
If you want to just extinguish their will utterly, then you want the Slave template. It's pretty much designed to fit that idea-the creature has no Willpower, and the only thing that can make it get weird on you is when external events, like Solar Charms, force it to rekindle the fire of identity.
Taking advantage of the demon's weakness, under all but the weirdest circumstances, is going to keep it from doing Virtue channels or spending Willpower for automatic success on the summoning rolls. This in turn lets you keep your summoning cost and chances sane. :)
Is the limit break a function of the spirit's nature, the binding magic or some combination thereof? Does the same spirit always have the same limit break?
Hm!
It's a combination. Basically, when you break their will, it's like breaking a vase: it's going to break somewhere. If you're a Solar Exalt with Craft Charms, you could probably draw a chart on the vase before breaking it of all the fractures and what pieces there'll be afterwards. (Assuming you know where and how hard it's going to be hit.)
If you then throw all the shards in a bag, and carry the bag with you, eventually one's going to work its way out and cut you. When and how isn't totally predictable, but it does depend somewhat on whether they're big pieces or little sharp shavings.
In short: it's as reliable as the Storyteller wants it to be. For an ecstatic, you just can't know; for a courtesan, if it's fallen into Lash of Self-Hatred every time so far, it probably won't fall into the Fury of the Scorned often, if ever. :)
What ways do mortal summoners, and sorcerer summoners use to shed unwanted demons in lieu of being able to order them to Malfeas?
I don't know. Kill them? For most templates, sticking them in a box somewhere and waiting for them to time out naturally also works. :)
Same way you get rid of any mind-controlled slave whom you don't want any more, really. :)