I am thinking that if you worry that someone steal your code, why do you want share your code publicly?
It's not true, OP already state he want to be owner of the code with MIT license over other copyleft license. It's up to other people whether they honor such license or not.
I do not know if there are rules in the forum regarding this, but I believe that once the post is published, it will become the property of the forum and is no longer yours alone.
Do you have any specific reason to think all posts created by user is property of the forum?
Such a question was raised about the rights of members and the rights of the forum when a member deleted a post containing good educational content. The objection was: Does he have the right to delete this educational content after he published it on the forum or not? But I think we haven't got a clear answer yet.
The forum provide delete button for reply on most condition, so i'd say yes.