If you are on Unix, you could use the access time of the files in there. Just enable access time for your filesystem, then run a clean build of all your projects you would like to keep dependencies for and then do something like this (UNTESTED!):
find ~/.m2 -amin +5 -iname '*.pom' | while read pom; do parent=`dirname "$pom"`; rm -Rf "$parent"; done
This will find all *.pom files which have last been accessed more than 5 minutes ago (assuming you started your builds max 5 minutes ago) and delete their directories.
Add “echo ” before the rm to do a ‘dry-run’.