Why do you want to do that?
The cache improves performance in both the reports and when using the HUD.
If you do want to remove it you will need to manually 'drop' the cache tables from your database.
You should not do this unless you understand what you are doing because if you remove the wrong thing you could damage your database beyond repair.
Before doing anything else, make a backup of your database
To drop the cache tables run Start > Programs > Postgres > PGAdmin III.
Expand the database structure to find the database you're interested in.
(See the backup link above for detail on using PGAdmin)
The cache tables are called "holdem_cache" and "tourney_holdem_cache".
To remove these right click them and choose Delete/Drop. NOTE: THIS CANNOT BE UNDONE so be sure you want to do it, and be careful not to delete anything else.