Well, you could just say, thanks but I'm not taking suggestions at this time. I will put up a suggestion box when I feel your imput is required.
Okay, that was flippant, but I've had certain people give unsolicited advice and I usually just roll my eyes and walk away from them. I guess it depends on who is giving the advice and whether it's truly out of concern or just someone being nosey. The nosey folks would get something like what I first posted. The truly concerned would get more courtesy, but the underlying message would be pretty much the same.