go to edit on your first post. should be an option to delete, if you delete the first post it should delete the thread. if that doesn't work there should be a delete thread button.. not exactly sure i know i've delete my own thread though before.
You have 7 days I believe from the time you posted the thread to delete it. After that: http://forum.grasscity.com/general-feedback/899395-grasscity-com-forum-guidelines.html