First thing first, I hear many people say:" ...that is ridiculous, no one will get back to me then..."
It depends on who you are and who you want to be for sure. If that request from a relative to design a logo for their band bothers you, there are two choices - you do it, or you don´t.
As soon as you waste time on that thought to help someone by doing a side-request for zero money (of course it´s your friend, aunt, daughters friend, relative, etc) you will get into an endless loop of doing changes for zero money. And you don´t show up professional as well if you don´t take money.
Tricky situation this is...
Sure thing is that declining just because you are an illustrator and therefore not specialist in doing logos is true and you know that. But as soon as you explain that to others it can lead to a different picture of yourself, one that shows you as not being capable to look beyond your shadow.
That can be bothersome.
The only solution is to offer help to help others do things on their own (at least if you know they won´t pay you anyway).
I do that in my profession as lecturer as also in personal life.
My father got to buy his first PC at the age of 60, now he knows how to burn CD´s, browses the web and can write emails - he also does his taxes online. And I´m really proud of him for having developed a curiousity to find out most of these things on his own.
When, as a professional, you want to decline a request, give an opportunity to people to come up with a solution on their own. This gives others the impression that you are a helpful person and you give an impression of what you would do if you were them.
That gives others a valuable standard, one they can live up to and be proud if you will approve.
Easy as that, isn´t it?