Mods didn't do that, when enough community members downvote your post because they find it in poor taste it automatically will hide the post and lock the thread if it is the OP of the thread.
It's an automated measure mainly to stop forum abusers before mods could show up, a relic from when there was more people around.
The fact you tripped it kinda speaks to how poorly timed the joke was, not to some conspiracy against you or free speech.
The problem with free speech is, you are free to say whatever you want... just you aren't immune to the responsibility of what you say.