When I'm riding my horse molly if the horse behind her Gets close (I mean even if a pony could fit in the gap) she will kick! What shall I do to stop her?

Then if it is just horses, it is all down to the horses relationships with one another. Therefore, it would be a good idea to put her out with the horses she rides with so she can assert her dominance over them there rather than when riding out.
She doesn't mind things or people it's just horses she hates
I agree with Sharon here. Make sure she is comfortable with things being close to her through desensitising.
...Or next time she kicks, put her straight into work and repeat until she realises that every time she kicks, she will just have to work. Make the wrong thing hard and the right thing easy.
Know that horses will always attempt to kick others if they're trying to show authority and dominance. Allow horses to get used to each other in a field before riding out with them to make the chances of kicks less likely.
Do a lot of ground work and desensitizing work with her. There is a horse just like that in our barn and she kicks if another horse is in the 2 meters of her. It might take a long time for her to stop this habit so tell all the other riders that you ride with what your horse does. This way no one will get hurt if your horse does kick. I wish u luck!
At shows ppl put red ribbons in there horses tail so the other ppl know she will kick
Tie a red ribbon in her tail.
Have you tried some desensitizing exercises?
You should put a red ribbon in her tell at shows and make it known she's a kicker. There's nothing you can really do
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