When you thank a fellow employee for his advice, you are actually rewarding the very behavior that you want to stop. Since “thank you” is a form of positive reinforcement, the behavior that generated it is likely to be repeated.
If you truly want your co-worker’s constant flow of marginal suggestions to ebb, you need to let him know. Importantly, you can do so and still be polite.
For example, the next time he approaches you with his suggestions, you should give him some businesslike feedback. Let him clearly see how his ideas do not apply to your work. After a few visits with this kind of outcome, he will be less likely to come around, since there is no gratification associated with doing so.
At the same time, it is rarely a good idea to totally cut off inputs of fellow employees. Sometimes their distance from your work can provide them with a fresh perspective, and this can lead to fresh ideas. And since turnaround is fair play, perhaps you have some ideas to help him do his job, other than suggesting that he focus more on his work than yours.
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