I think you deal with bad designers the same way you deal with bad developers.
Try not to hire them in the first place, fire them if you can, and if you're stuck with them try to make sure they are occupied with tasks that will not cause too much overall damage.
The horror stories I hear are usually when a Graphic Designer is given the role of a UX/UI designer. There is nothing that is going to quickly fix that situation other than getting them some much needed education and mentorship.
or use "ugly arm" method (I don't know if this is the correct English name of this). Ir, show to they two options. First a obviously ugly option that nobody would use. Then, show your desired option. They would use your desired option, perhaps with some minor changes, and they feel that did something.
Not quite the same thing but I've heard of people putting "A Duck" in their designs, which is a superfluous design element that is included solely for the purpose of letting a micromanaging superior remove it so that they can feel like they contributed to the design.
https://blog.codinghorror.com/new-programming-jargon/
But whether it's a duck, anchor, strawman, whatever you call it -- the risk with proposing an obviously bad solution to make the good solutions look better is that every once in a while they'll pick the bad solution. I think it was a talk by Mike Monteiro where he explicitly cautioned against using this approach because it tends to backfire hilariously at the worst possible moment.
Try not to hire them in the first place, fire them if you can, and if you're stuck with them try to make sure they are occupied with tasks that will not cause too much overall damage.