Firstly, you should try to control your laughter. The Prophet (SAW) said: "Do not laugh too much for it deadens the heart." This is not to say always be serious; we know that The Prophet (SAW) had a sense of humour and used to smile often. However, while he joked, he never lied, and he certainly never mocked people.
Secondly, backbiting and slander are haraam and you should not indulge in them. Unless Allah chooses to forgive the one who backbites, for them is a harsh punishment in the hereafter so you should refrain from it.
Thirdly, when a believer is confronted by an obstacle, such as an uncontrollable desire to laugh, he should make du'a to Allah to remove this from his character. Don't ever underestimate du'a: Muhammad (SAW) called it "the weapon of the believer."
There is also a Hadith Qudsi: " Allah says 'Oh the children of Adam! Until such time you invoke Me and have trust and hope in Me, I will continue forgiving your sins no matter what they are, and I do not care of anything (i.e. He is Independent.)'"
Finally, you should re-assess your sense of humour. I know things like that may seem funny at first, but when you really think about it, what you are doing is insulting the a creature whom Allah, the Designer and Creator, had designed and creating. It is laughing at what Allah has made.
And Allah knows best.