Using of autoComleteExtender

Hi,

I'm using autoCompleteExtender and I want to know how to eliminate users from typing words that are not in the suggestion list. For example if I type in the textbox 'ab' I'll get this list for choosing : 'abc', 'abd','abe','abf'. How to ensure that the user choose one word from this list and not typing other word that not include in the list like 'abdfgh'.

Thanks,

David