Several Facebook features exist with the primary intent of making your user experience more personalized. For example, the Facebook sidebar suggests that you "Like" Facebook pages several of your friends have liked. Although it isn't possible to turn these suggestions off permanently, removing these pages as they appear prevents each from appearing again.
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Hover your mouse over the "Like" suggestion you want to remove as soon as you see it. If you navigate to another page, you have to wait until the next time it shows up to remove it.
Click the "x" that appears to the right of the page name to prevent it from being suggested to you again. If the page was a sponsored ad, click the reason you don't want to see it -- "misleading" or "offensive," for example -- which provides feedback to Facebook as to whether it should use the ad in the future.
Continue clicking the "x" buttons next to pages you want to remove from your suggestions until no more appear. Over time, Facebook replenishes the list of pages it suggests to you, but frequently removing them once they're added prevents you from being bothered by them.