There are two states of "used" memory, "inactive" and "active".
Active memory is memory that is being used by a particular process.
Inactive memory is memory that was allocated to a process that is no longer running. are pages which have not been accessed "recently"
to see the memory use /proc/meminfo rather than vmstat -a
cat /proc/meminfo
you need not to clear this Inactive memory When system reboots this memory automatically became vanish, still If you have a large amount of inactive memory ,you can use following command.
free && sync && echo 3 > /proc/sys/vm/drop_caches && echo "" && free
Edit
As per @kubanczyk comment:
you can find more information from this question
