You don't have to respond. You just have to acknowledge that you've received the request and that you still wish to hold the copyright.
How about you pay me to respond to you instead? At copyright attorney rates for my region. You send my attorney a request and your billing information, and he'll reply to you on my behalf and send you the invoice for his time.
Otherwise people will be able to cause huge financial impositions by essentially DDOSing people with copyright requests. You want a guaranteed response, you pay for that response in advance, otherwise kick rocks.