Hi newcomer and welcome to the Tom's hardware forum.
It's a simple process:
1- Go to your thread, look for the "EDIT THIS MESSAGE", NOT on "quick edit".
2- Look for the option "DELETE THIS THREAD" to the end of the edit box of your thread.
3- Check the box and select submit.
Thanks for your answer but the process did'nt work, I tried twice to delete the whole thread but the message " you are not allowed to edit this message " appear in a new page.