How to teach dog to stop humping?

My dog is really bad at humping. He did it to us maybe 2 times when he was little but that was when he was the only one and he was so worried out us liking him that after we got mad at him for the 2nd time, he stopped (I ignored him for an hour the second time he did it and he never did it again). We have a girl now though and shes not going to ignore him so he wont stop. We're saving up the money to have him neutered right now (he weighs 120lbs and hes so big that its going to cost $240 to have him neutered). She turns around and snarls and barks and growls like shes going to bite him when he tries to hump her so its obvious that she doesnt want him on her but she just makes noise, no biting, and her threats are having no effect on him. She makes a lot of noise too and you can hear her down the street so they cant play outside for long. He had basic training when he was little but he didnt get far, he just picked up a halfway understanding of sit and only does it when he wants something. The trainer couldnt even get through to him so he really doesnt do good with commands. He's 2 years old, if he couldnt learn it at 4 months, i dont think hes going to pick it up at 2 years. How can I get him to stop humping her? Soon it wont have any "permanent" effects but i want him to quit. He has learned to not do it when he thinks we're not watching him though -_-'