And, why would you want to do that? The idea behind an online forum is that threads which are posted remain visible forever. This way, others can learn from your questions and the answers which result.
"Lisa, in this house, we obey the laws of thermodynamics!" - Homer Simpson
"I have my standards. They may be low, but I have them!" - Bette Middler
"It's a book about a Spanish guy named Manual. You should read it." - Dilbert