We call it Scope Creep here, and it is one of my biggest challenges running software projects.

Just 10 minutes ago (which is after hours here on a weekend) I had to explain to a client that they needed to have the actual people that are going to use the software test it, before starting to call me with 'refinements' that they want changed before the actual user ever sees it.

Often enough, this particular client has had us make changes that they later asked us to revert do to user complaints (much like your example) ...
Unfortunately, sometimes, it takes a letting them make a few of these mistakes first (to show them they need to trust your opinion) before you get their buy-in.

Most of the time this ends up saving the client money in the long run .. just telling them (gently) "your opinion doesn't really matter in this case, and we choose to say no", works once they realize you were right before.

Every client is different, but I been there more than once ..

I do not work here, but the benefits are still awesome
Make your sound your own!