I used to ALWAYS respond to every message or request I was sent..
Even if i hated the concept/the photographers creepy introduction message and his work.
I would say "no thanks for now, if I ever feel like updating my portfolio with those concepts I will be in touch"..
Skip on two months, another message from the photographer "Are you still interested in working with me?". Again I would reply being polite - "Sorry I'm a little busy at the moment and still not really looking to add what you photograph to my portfolio"..
Then skip on another month - Photographer messages again "Hey I'm going to be in your area on Tuesday, how about that shoot now?".
It goes on and on and on, is it more polite from the get go to say "Sorry your work is so crap that I wouldn't even work with you if you paid me/ your initial message is nothing but creepy and I wouldn't work with you because of that message or... / sorry your concepts are bloody awful".
I think its much more polite to just ignore those you will NEVER ever work with.
It wastes less of their time - doesn't batter their ego in the same way if I was brutally honest and it doesn't waste my time.
I've only outright ignored somebody once btw.. Other people I have ignored after months of me trying to be "polite". Hell even me saying "sorry its not really the style which I want to do" doesn't get the more harass'y photographer to piss off. They will come back with "but I think you would be perfect for it". So sometimes you either have to be completely blunt or in extreme circumstances ignore them, its far nicer to ignore than to say "look your shit".
I know its not nice being ignored, however it happens to everybody.
I once made the mistake of posting a tf casting on a website and out of about 50 replies only one of them was anywhere near what I was looking for. I asked for something different, not necessarily nude and all but 2 of them suggested nude concepts, many which were above my level and the quality of the photography was well below par. Most were messages from people wanting me to stand naked beside a tree - which is fine but I have done that so many times now that its not something I want on my portfolio. One was even lazy enough to copy and paste his "ideas" from his profile notes which included - pregnancy/tattoos/petite/plus size/mature women concepts which clearly I wouldn't have been at all suitable for.
I don't reply to castings often at all, I read them throughly and also look at the photographers previous work/references and their profile notes. Then I will message them with a personal note to state my interest and usually I make it clear I have read their castings and their profiles. Its rare for me to be completely ignored though, generally if I put the effort in and form a well thought out message then the recipient responds.