My opinion is that people try to make themselves feel better by making others feel smaller: There are a lot of insecure people on the Internet. It is easy to make fun of people who are learning another language, because everyone makes ludicrous mistakes. It takes a bit of courage to risk publicly embarrassing yourself in a foreign language, but for better or worse it is the only way to learn. I also think it is worse for women; men feel that they can take advantage. Unfortunately, there is not much you can do about it, except that I think you can block them or complain to the site administrators.