Originally Posted by flarmip

Indeed. The definition of IDB is:

So the key test is whether there are "reasonable grounds to deny ... boarding". The list is clearly not exhausive, but equally in keeping with CJEU jurisprudence one would expect any derogation from the general right to IDB compensation to be interpreted strictly.

Therefore I would say it is highly situation and fact dependent as to whether refusing to unnecessarily check-in hand luggage would constitute "reasonable grounds to deny ... boarding". I don't think a blanket answer can realistically be provided.