Not every potential client who walks through the door is worth the trouble. There's the controlling "my uncle is a lawyer and he said..." client. There's the "can you bill me later... at a lower rate" client. And, of course, the overly specific herpes defense client.
When you start out, you'll be tempted to take every case that walks in the door. First of all, don't. And as you get further along in your practice, not only will that feeling subside, but you'll get better at learning when to say no, and more important, how.
For now, here are a few ways, each of which is inspired by the many times I've been rejected, for softening the blow while still getting the message across: