People that are at the barn to ride tend to do the minimum socializing needed to just glide along and get to the horse. People who are at the barn to socialize tend to do the minimum riding needed to hang out and chit chat. Two completely different cultures. To you, they look like dithering fools. To them, you look curt and abrupt and in need of being included and mother-henned into participating in the barn sewing circle.
Lots of times I’ve been safely in my stall grooming or something, and listened to the chitchat of people who have to process everything and dither over it and rehash and rehash. Or “manage” all their social contacts every day. Even if it doesn’t involve wrong-headed ideas and unsolicited advice, it is extremely tiresome if you are there to get things done, and I try to duck it.
So I wonder if the stupid advice is people trying to interact with you and pull you into the barn social norms. In that case, perhaps if you give them a deliberate, but measured, amount of interaction when you get to the barn, like some loud cheery empty talk about the weather or the great new brooms the BM bought, or the traffic, or whatever, then say gosh! I gotta really rush now if I’m going to get all my horses done today! Or think up little questions for them, or little favors to ask, or compliments (your horse has such a pretty tail! is always a good one). Feed their deep need to have everyone “in the herd.” Then go do your own thing.
If they follow after you and offer advice, then say one of the excellent rejoinders the other posters have offered 
Make fairly frequent reference to “my trainer says.”
The thing is, among people who are into “alternative health” in a dilletantish way, a perfectly normal conversation is for them to sit around saying “oh you should take eye of newt.” “Oh, wow, I will, I’ve been taking tongue of dog but it hasn’t really helped.” “Eye of newt, absolutely, with camomile tea. It totally changed my aura.” Etc. Etc. Etc. I don’t know if anyone ever even takes anyone else’s advice. It is, I think, a way of communicating benevolent intentions in a formulaic fashion. It is also a way of sounding like you are concerned about the other person, but really just talking about yourself. It’s the verbal form of re-posting “memes” on Facebook. It is the social gesture minus any real content.
So is talking about the weather, or how pretty your horse’s tail is, or the new soap in the bathroom. But maybe you can supply a new, less irritating, topic for the empty social gestures.
However, this is all very different from the kind of person who is offering advice and really wants people to think she or he is an expert. Or who is offering advice in an effort to make you look or feel stupid. Those are generally people who have a few (but not quite enough) accomplishments to their name, and want everyone to know it. Those are also the people who will get hostile if you challenge them or don’t follow their directions. My guess is that you would get proportionately more of those people at show barns, which draw competitive people.
You might have someone like that at your barn. But it’s just as likely that the stupid advice is not really meant to challenge you, but rather as a way to pull you into conversation.
