We would need to know what you believe has deleted the items. The reason I ask this, is it may be the post before register feature itself that has deleted them. The process is
User posts
They are asked to register
They enter their details and are sent an email
if the user registers, it turns into a post for that new member
if the user does not register, that post is deleted after a period of time.
So as you can see there, it may well not be being deleted by any spam filter. If it is, we need an example of x post being deleted because of y criteria specifically so that we can advise. At present you are mentioning spam filters on post, which simply dont exist.