You can edit posts and thread title.
If it needs deleting ask a mod or admin and they may delete it for you. (depending on circumstances)
EDIT: I will modify my post seeing as you did to yours

I think it has been discussed before but it was decided that some people may abuse the button.
Eg: if you ask for an opinion on a bike and everyone flames you then the easy way out is press delete, when you asked for comments and criticism etc.
Correct me if im wrong though.