Many popular Internet search engines will try to guess what you are searching for based on the first few letters you type. Google calls this feature "Autocomplete" and Yahoo! calls it "Search Assist." Bing and Ask.com both call them "Search Suggestions." Not all search engines allow you to disable this feature. According to a Google employee, you can not turn off Autocomplete because all Google users are meant to have a uniform searching experience. Yahoo! does let you turn off Search Assist.
How to turn off Suggestions for Yahoo!, Bing, and Ask.com
To turn off Yahoo! Search Assist, sign into a Yahoo! account. If you do not sign in, you can still turn off Search Assist, but it will not save that setting. Signing in will ensure that Search Assist is always turned off for your account. Turn off Search Assist from the Yahoo! main page by clicking the arrow under the search box and and choosing "Off." You can also turn off Search Assist from a results page by clicking the arrow below the search box, clicking "Settings" and choosing "Never" next to "Select how often Search Assist results are shown."
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To turn off Bing's search suggestions, click "Preferences" in the upper right corner and uncheck the box next to "Turn suggestions on."
To turn off Ask.com's search suggestions, click "Settings" in the upper right corner and check the box next to "Show me Search Suggestions."
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It is not possible to delete individual search suggestions because according to Yahoo! Help, the suggestions are not based on you or your personal search history. The suggestions are made based on what all internet users have found helpful in the past.