What do you do if the client chooses a bad design option and ignores the designs offered by you?

If a client chooses a bad design, the best method to follow is to give him an analysis of it and why it is bad. The client does not have the knowledge which the designer has, so it is the designer's responsibility to educate the client. And if the client insists on the bad design, it's the designer's decision to whether resume working with the client or not. At the end, the work created will be an addition to the designer's portfolio and may harm him more than the financial benefit.

Use the tools of work that I know about such a design, to convince him to change his mind.

Simply agreed with him but do my tricks to make it better at least to be satisfied for my work

Basically designer does3 good options, the intent here to discover3 different solution/paths to open a solid discussion with the client and achieve a good results. once the client had selected one; sure you will sigh deeply.

Good answers from my colleagues 

Thanks

Know the rule customers are always right :)

there are some client and bosses chooses those not so good in your eyes those design had made, rather than chooses the design that your pour a lot of effort.. what a big sigh.. after all done.. the best thing is know the nature of business/organization of the client .. the boss concepts and it's flows of business..

The satisfaction of client should be the first thing in our mind. If he selected a bad design try to understand his mind and create accordingly. Of course it will be very disgusting for a designer if somebody reject the good piece of creation. 

I agree with Mahmoud Hamam's statement and I'd like to add that sometimes "a bad design" is subject to taste. After all, that which one person finds beautiful may not appeal to another. If you absolutely feel uncomfortable with the clients choice, I would say communication is the best remedy.