Among many other awesome features, our USU mod does exactly what you'r asking.
Check it out.
phpBB SEO Ultimate SEO URLphpBB SEO Ultimate SEO URL V 0.6.0
You'll get something taken care of that's been bugging you and give your forum a great SEO upgrade.
Think it over.
p.s. If this is a new site then I suggest starting from scratch with our phpBB SEO Premod V 3.0.5
which includes all the mod's we offer here pre-installed in a new phpbb-3.0.5 install package.