I posted company website on posts for coding help, but do not want clients to find these pages when searching for our company on search engines...is it possible to delete my old threads or have them deleted?

Only Mods and Admins can do that. Try PM'ing someone on this page:
http://www.webmaster-talk.com/showgroups.php

You cannot close a thread, but you can edit the text you wrote.
You have an small "edit" button in the lower right corner of your post.

You can always edit the things you wrote, so you don't need to worry too much about it but do it as quickly as possible before any client see it.