Yes, forgive me for buying a tree on Craigslist and not studying bonzai for at least a bachelor's degree in tiny trees. And if I wanted a bonzai that would be 3 feet high in six months I would have bought a rose bush. I know that there are serioius controls that can be applied.... leaf pruning and root reduction. Not on this forum though.
LOL, welcome to the nut house. This is precisely the place to be for bonsai guidance. Just always remember that a clear picture of a fuzzy scene is a fuzzy picture and you'll be well on your way.
Trees grow, that's what they do. We take advantage of the tree's natural growing tendencies to develop bonsai and, once we have the "finished" bonsai, place it into a small container to preserve what we developed. But trees are living things that don't really care about our intentions for them. They will always do what they do - grow. So we cut them back to preserve the style of the tree. Didn't you ever watch Karate Kid and see that Mr. Miyagi is always cutting something? Those who have larger leaved trees, like red maple, will need to defoliate to reobtain small leaves they had in a prior year. There's no way around maintenance. No reason to be surprised that an unpruned stick in the dirt grew into a 30 foot tree.
The way to get them to stop growing is to kill them. Otherwise they will grow. Less so in a tiny pot crammed with roots, but grow they will. And when they stop growing, you need to repot or they will die.
But before you do anything, you need to know what you have. Some think it's a tropical and others an evergreen. That's like asking how to care for a pet and one person advises you how to care for a fish and the other a dog. Without knowing you're either going to drown the dog or take the fish for a walk. Neither will work out well. Hey, at least it will stop growing.