I'm going to disable the recline on the plane seat ahead of me
What makes you want to disable the recline feature?
I don't want them leaning back into me.
That is mean of you to do. It's rude to make someone else feel uncomfortable while traveling on a plane. You should be nice to others so other people will be nice to you.
Why I should be nice to people?
You should be nice to people because it's the right thing to do, and making it so the person in front of you can't recline their seat is not very nice. Everyone would be much happier if people acted more kindly to one another.