You can see the [X] icon at the top right corner of the image below
If you click on [X] icon, you will get a option to customise your ads
Generally Google displays ads based on the search queries, but it also considers other parameters like your age, name, location, interest, sex and the language you speak which it gets from your Gmail account.
In the ads settings just select different parameters like your interests, language etc. After that Google will display only those ads which you really want to see.(But Google does not guarantee it!)
If you want to view the ad again then you can click on the "UNDO" link. Same ad will be displayed again.