Guarding the Post

[Chippy4463]

Well ive been a lurker for awhile and have a question that I cant seem to find the answer for regarding Guarding in the post. I referee for an after school club for teenagers and there is one guy who is pretty good in the post. Well lately to slow him down defenders have been using there forearm as sort of a barricade to stop him from backing down, imagine trying to hold a door closed from numerous people trying to run in..Yes like that. now the post player says its a foul but the defenders say its legal.. Ive never seen anything like this but, figured somebody could shed some light on topic for me. It will be greatly appreciated by all parties involved. What is allowable for a post defender ?