I think you are missing the concept of Patterns. Every Sphere rules over a Pattern of those Spheres, wich means that Forces only affects Forces Patterns, Matter only affects Matter Patterns and so on. You can't affect Matter Patters with Forces, that's it.
That's the way you stop a player doing whatever she wants with the Spheres, the Patterns.
There are some cases when you can do the same effect with different Spheres, but that's another thing. It's a different aproach for the same result. For example, you can fly with Correspondence 3 or Forces 2. The first bend the space around the mage making her fly, the second manipulate gravity to do the same. Another example could be manipulating feelings with Life 3 or Mind 2 (or both, for more fun!).
In closing:
Do you want to teleport a bullet straight on the face of a thug with Correspondence? Great. You have to teleport the bullet's Matter Pattern in order to do so. You don't have the Matter Sphere? Too bad.
Do you want to accelerate the atoms in an object to make it burst into flames? You need to manipulate Matter Patterns AND Forces Patterns. Or, you use Forces 3 + Matter 3 to transmutate Matter Patterns into Forces Patterns. Or, Forces 3 + Prime 2 for the classic fireball ftw.
I don't know about Second Edition but Third and 20th Anniversary Editions explain the Patterns quite well.
Another thing worth of mention is the Paradigm and Foci, as you and other users had stated before.
A Dreamspeaker can't just bend a spoon with his mind just because she has Forces 3. A Dreamspeaker will use her foci, based on her paradigm to use Forces in some way or another, like a chamanic ritual or a pact with spirits of nature or anything which makes sense with the specific character's systems of belief.