Since we use phpBB, this is a sort of suggestion for them at their forum!
Have you tried using
* as a wildcard for partial matches (of either posters or specific words for which you are seeking)? Or using the + and - to further define your searches?
For instance, this search brings up 930 results...essentially posts where I have mentioned "banana" without any mention of "munching" or "munches" within the post:
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While without the -munch (and thus searching just banana) I get 1437 results.
Remember, you can also limit your results to a specific time frame if you when you want to look for results. Here is might first search again (-munch* banana), limited to the last 2 weeks. 8 results are brought up:
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--Andy