Because of the incremental style of that dialog's searching, it kind of needs to always do a prefix match. You can't type the whole word without typing the prefix first. This way it guides you along, otherwise, you wouldn't get anything until you had a whole word match.
However, you could do a regular expression search and surround it with ^ and $ to force word boundaries.
Also, if you keep the list sorted by symbol name, exact matches will always be at the top.