Use data and case studies to back up your position
It's hard to tell a client that an idea will not work, but approaching it correctly is a huge step in winning the battle. Here's why thoughtful case studies and meaningful data are polite ways to disprove a bad creative suggestion.
Give the client multiple options for measuring your idea
Brands are generally obsessed with CTRs. This can lead to ideas that are one-dimensional and centered around increasing the clicks. If you have a new creative idea, give your client other metrics to show how you will measure success. Exciting alternatives can move the focus away from the CTR.
Remind the client that you are the expert
There's a fine line between confidence and arrogance. But, at the end of the day, you are the expert. Don't forget that your client hired you to make creative decisions, not just take orders.