How to defend against optimized charge?

[Cyrion]

Isn't there a spell somewhere that conjures a 5' box or something as an immediate action?

More generally, battlefield control. The charge mechanic is, as Curmudgeon pointed out, very fragile. Almost any Conjuration (creation) spell has the potential to become a wrench in its works.

You may be thinking about Blockade from Complete Scoundrel- it conjures a 5' block of wood. However, it's a swift action, not an immediate action.