I think I've been lucky to find respectable readers, so I have not have this problem but...

For my next web comic, I can tell that I will have more people 'requesting' things and I know about what too...

I just pretend I never saw/heard the comment XD If the person is next to me, I act like I was so absorbed in the comic that they just leave and go do something else.

For online comics, I tend to not 'reply' directly but I do a general blog like thing for my next update and just not include those comments.

I usually work on my pages when I'm in class or hanging out with my friends (yay, multitasking), so I tend to get a lot of "hey, you should put ME in your comic" or "why doesn't she have any hair (yet)?" kind of comments. When it gets annoying I just say "Uh, because." in just the right tone of voice and they leave me alone.

I do ask my friends for suggestions a lot when I'm stuck, though, and usually we manage to come up with something that works well together. When they get stuck on/convinced of an idea that I don't like I just explain why it wouldn't work with what I already have planned or because the character wouldn't do that, etc.

Of course, there was one time when I was, like, 17: I was drawing during school and this girl didn't like what I was drawing for whatever reason, so she grabbed my paper and my pencil right out of my hands, said, "no, it should be like THIS," erased part of my drawing and drew what she liked better over it. SO many things I could/should have said, but I was a little too stunned, so I ended up just grabbing it back and erasing what she did.

(I've had the same experience like you, Miruku. :( It's like they don't respect what we're doing and it usually pisses me off a great deal. Eek.)

I had a friend who insists I draw 'like this, this and this' instead of 'that and that and that' and whenever I say it won't work, she would always say things like, 'But it's nicer THIS way!' Of course, if there were good suggestions, I would take it up but at times when she does that, I feel really uncomfortable because I'm not really good at saying 'no' to people. ^.^; Usually, I end up saying 'okay..' and does things her way and then do my own version at the same time.

Now, I don't tell my friends that I'm doing a comic. ^.^;