There's a difference between meddling/interfering and making suggestions. There's also a difference between valuable suggestions and useless ones.
If you're being actively interrupted while working on code, that counts as meddling or interfering. In such a case, you have every right to get the salesperson out of your face. Additionally, if your decisions are being undermined by constant second-guessing at meetings, if the salesperson is making an effort to undermine your authority or question your competence, that's certainly unacceptable.
However, if you are simply hearing lots of opinions when you present something, this isn't meddling. It's just unwelcome advice, like you'd hear from a friend who sits on your deck furniture drinking your beer while watching you teetering on a ladder while you paint your house.
It's quite possible that the salesperson is well-meaning and just wants what he/she thinks is best for the customer, even if the suggestions sound silly or capricious.
By making suggestions, your salesperson may be hinting at weaknesses in your design or interaction model. Or they may just have their own personal axe to grind.
If there are solid justifications for changes, it's worth trying to find a way to respond to feedback, even if your actual implementation doesn't end up literally incorporating the specifics of the suggestion; it's quite possible that you'll learn something if you listen. Just keep in mind that the words used in those suggestions may not refer to the actual problem; they're just a proxy to an underlying issue that hasn't been adequately understood yet.
I've sat in meetings where an executive wanted to fiddle with how many pixels to the left some control was instead of discussing what business logic needed to be there, and we certainly found this not a good use of his time or ours. But I've also listened to people complain about the look of something or what type of control was used and it was really a roundabout complaint about how something worked, not what it looked like. Keep in mind that users usually don't know what they want until they've gotten it; if you think of your salesperson as a proxy for an user, there's a good chance he suffers from the same problem.
So don't feel obligated to accept every suggestion, but don't be so quick to dismiss every word coming out of your salesperson's mouth.