Ok, ice prison is essential for yuki's to stop opponents from moving, to either get a perfect hit with 1k needles or hit em with ice mirrors.
I use ice prison all the time, and it sometimes happens that my opponent can substitute out of it.
I wanted to be sure and checked it with King, but he couldn't substitute out of it.
However, later on it happened again... I just caught someone in it and before I hit him or it broke off on its own the other player poofed out of there.
Also, I've never seen shadow clones stay in ice prison after getting hit by it. they run right out of it.