How do I limit the vertical height of my girls.

I've got a small wardrobe i'm using for the grow and right now they are vegging at about 4" from the 400W air cooled HPS light.
I'm putting them to flower this week, I should have put them to flower a lot sooner but this is the first time for me.
I've only got about 6" clearance to the top of the wardrobe so I'm looking at cutting the plant tops to keep them from growing up anymore, is that a good idea?
Any other suggestions to keep them from growing taller?
Ive included a pic.

pruning is good but when you do you need to veg for at least 1 week before flowering.you will double you tops good luck your plants will double in height wen flowering. and you cane pull the canopy down with string to.