Hi Yves
With respect, that is a very bad idea. Unless you really know what you're doing, you can cause all sorts of relational integrity problems within the database, which in turn can cause exceptions within the Workflow engine.
We have a very early version of a LiveCycle purge tool currently in development. If you're interested in this product, please send an email to info@avoka.com.
Howard
http://www.avoka.com