how do you trim back, if possible a fornium plant ??

Do you mean Phormium?
If you do, then cut leaves off low down then basically use either a spade, a sharp knife or a saw to divide your plant back to the size you want.

They are tough old things. If it is a Phormium.

You could give the sections you seperate to friends or family.

I have a large Phormium and cut chunks off it a couple of years ago to try and reduce the clump size. Unfortunately it is now back to the same size as it was before I did anything, so you may have the same problem again soon. Your only choice may be to remove it and replant a small piece further from the window.

They are thugs, they do grow quick. I bought one New Zealand flax, divided it into 3 of which two might be getting chopped down and dug out this year. Look nice when small to medium unless you have a huge garden.