Print Without Ads is a small add-on designed to remove advertisements and other Flash objects prior to printing. Simply use File->Print Without Ads or add the Print Without Ads toolbar button.
Note that Print Without Ads may not block all ads - it uses a mechanism to remove ads on the fly, instead of while the page is loading like in other ad blockers. This method isn't as effective but otherwise shows ads when you aren't printing.
If you would like to have a more comprehensive ad blocker that always blocks ads, try Adblock Plus.