Clients are notoriously picky, but as a freelance designer, you're constantly subject to their changing whims and hair-brained idea. That's not to say that every piece of customer feedback is useless, but as freelancers you're usually left feeling something like Dogbert.
So what can you do if a client is insisting on a terrible design? There are a few things you can do to move things back into the realm of good design in these situations. What tactics you pick depend largely on your understanding of the customer. Choose wisely, my friends.
1.) Remind the client that they hired you because they were happy with what they saw of your previous work. By hiring you, they were admitting that they needed help, and that they were not skilled at creating good design. You are-- it's what you do. Therefor, a little trust is in order. Saying this with a smile on your face and a laugh in your voice helps soften the blow, and lets them know that you're being good-natured about their request.
Best for: Designers who have built a good rapport with their clients.
Worst for: Clients you have not worked with before, or with whom you've have had only a short relationship.
2.) Respectfully offer to withdraw from the project. I've only had to do this a couple of times when clients were insisting on changes that went against everything I held dear as a designer (think Comic Sans and 90's textured backgrounds). I always gently explain that creative differences are common, but that you take great pride in your work and wouldn't want to put your name on a design you didn't believe in.
This tactic is really a semi-bluff. Of course you'd like the money they're offering for the project (or you wouldn't have taken the work in the first place), but reputation is a huge part of your success as a freelancer. If you lose the client, you may have lost some money, but your reputation stays intact and you may have fewer headaches from dealing with a client who is obviously going to be difficult to please.
Best for: Designers who don't need the money from THIS client to be successful.
Worst for: Designers who can't afford to have their bluff called, and customers who would take great offense to being snubbed.
3.) Ask the client to sign a waiver stating that the design they've requested goes against your suggestions, and that they understand that by accepting the new design they're pushing they're waiving their rights to pursue any compensation or reparations from you if they change their mind in the future. You're protected from them trying to sue you if another trusted source convinces them their design was crap while also sending a message that you have very serious reservations about their designers.
In reality, you should have something along these lines in your standard freelance contract, along with an agreement that you will be paid for work completed (as either a lump sum or at a per-hour rate) whether or not the client accepts your final design or not. Pointing out this point in the contract is an acceptable alternative.
Best for: Designers who don't want to lose face in the design community, but can't afford to lose THIS customer.
Worst for: Clients easily put off by legal waivers.
4.) Offer to change the design free-of-charge if user testing determines that it was a dud. This is a good opportunity to do some early A/B testing with your mockup vs. the customer's requested choice, and to get objective data backing up your position. Use this option only if you 100% believe your design is where it needs to be, and if you're willing to work for free if your design loses out.
Best for: Designers who create solid designs and are confident in their testing methods.
Worst for: Clients not easily swayed by statistical information.
5.) Just do it. Look, we know that you don't want to just bite the bullet and complete work you believe is crap. Keep in mind, however, that especially early in your career, when you're hurting for money or experience, being a "Yes man" can be a good way to get some money in your pocket and generate a few referrals. Chalk it up to a learning experience, and resolve to get to a place in your career where you can back down from these situations in the future. Use this only when all other methods of recourse have been exhausted.
Best for: Designers who have no other choice.
Worst for: Designers who have no other choice.
I hope this helps give you some ideas about how to combat this in the future. I'm curious what the rest of the community thinks about this as well!