If you're trying to send a private message, then you won't be able to do that until you've made 3 (I think) posts - and posts on some boards, including this one, don't count.
Another option is to use the Report to moderator link at the bottom of any post in the thread. I don't know if that also requires you to have made 3 posts.
Having said that, RootsChat doesn't generally delete whole threads without good reason, so you would need to put your case to the moderator. If it's because the thread contains incorrect information, the best way is usually to post the correct information and explain why the previous posts were wrong - that can be a lot more helpful to future researchers than simply deleting the lot.
