I agree with ray ... that it's a physiological problem.
First issue is to find out
why she kicks. Is she in pain? Is she impatient? Does she dislike confinement?
What's the filly being fed? Lots of grain or lots of forage? Does she have companionship? how does she react to other horses? Can she be fed outside rather than inside? (nothing to kick when waiting to be fed)
At a farm I was at once, 3/4" rubber stall mats were hung in the stall of a confirmed kicker (was a mare too). The mats were hung about 4'4" from the floor - horizontally. You can either cut holes in the mat and use chains to hang them or use super large lag bolts and giant washers to attach to the walls. Leave the bottom edge loose.
Had a friend's mare that I had trailered many times before. In a 2H straight load (with my gelding and a different gelding) and in a 3H slant. In the slant, she had always been by herself. One time my friend and I were heading for a weekend of camping and riding, so I loaded her mare into the middle stall and closed the divider. Then I lead my QH gelding in (who she has been pastured with once in a while) and she freaked out - starting kicking the bejeepers out of my trailer. I don't know who got out faster, the gelding or me
Tried again. She was worse. She started squatting and peeing and flinging urine on the ceiling (7'4" trailer, horse was less than 15hands), on me, all the walls, etc. NASTY. Got her out and turned both horses out in the round pen. The 2 horses ignored each other. Tried loading her again. Worse than the 2nd time and the gelding wasn't anywhere near the trailer!Apparently, this mare had been bred before and being an arab must have been put in breeding stocks. We think it was the combination of the 2 dividers (like the stocks), her being in season, and who knows what else.
Took her out. Washed the inside of the trailer (because now, my gelding was telling me there was NO way he was going in there...). Loaded my gelding 2 times and let him chill out in there with a few treats and some hay. Unloaded him and turned him back out on pasture.
I then loaded the mare and took off right away. When she started kicking, I either slammed on the brakes or turned sharply (GN trailer) to knock her around
just enough to make her stand still. She settled down after 3 tries at kicking. Drove her 30 minutes home.
This is where a shock collar and being hidden can help. the instant the horse kicks, ZAP.