There is not always a clear distinction between physics and philosophy.
Interpretations of quantum mechanics are not empirically testable, so they could be considered philosophy, and yet the people who work on this kind of things are typically physicists who publish in physics journals.
There are people who work on the philosophy of science. For example, Norton's dome was originally brought up in a philosophical critique of the notion of causality,[Norton] but a lot of the literature that came after Norton's original paper smells more like physics or mathematics [Laraudogoitia 2012],[Korolev 2006].
People's opinions on what constitutes physics are themselves philosophical stances. For example, many physicists have attitudes that could generally be described as operationalist, and these attitudes influence what they would define as a valid physics question as opposed to a philosophy question. But operationalism is itself a philosophical idea.
For these reasons, I don't think it's practical to prohibit philosophy questions on the site, if they're also physics questions. If there is very little scientific content and it's mainly philosophy, then we could certainly suggest they move their question to philosophy.SE. There are certain topics that are basically FAQs where people imagine that something has a conclusive physics answer, but in fact it doesn't. Examples are the block universe and interpretations of quantum mechanics. This can be handled simply by writing good answers that say exactly that. For example, if someone is under the illusion that physicists actually study whether the block universe concept is valid or correct, it's only a physicist who can authoritatively tell them that this isn't what we study.
Korolev, "Indeterminism, asymptotic reasoning, and time irreversibility in classical physics," 2006, http://philsci-archive.pitt.edu/3003/
Norton, "Causation as Folk Science," http://philsci-archive.pitt.edu/1214/
Jon Pérez Laraudogoitia, "On Norton’s dome," Synthese, 2012 http://link.springer.com/content/pdf/10.1007%2Fs11229-012-0105-z