Copyright law differs quite a bit from country to country in that regard.Pretty sure they do, T&C's of this specific site & general legal precedent.Additionally, I am surprised that postings when added don't become the property/copyright of access-programmers.
In general, it is very unlikely that just the T&Cs of a website are a legally sound basis for a transfer of a copyright.
The likely legal situation is that by posting on the site the author of the post grants an indefinite right of usage of the posted material to the party owning the site. If and when such a right of use can be revoked depends on the applicable law and the T&Cs and may result in complex legal proceedings.
All legal matters aside, in my opinion Jon did the right thing in deleting the posts as he was asked to.