While purging the SOA Suite dehydration store is available through PL/SQL packages. It is not straight forwarrd. Sometime you ant to cleanup the whole dehydration store. Just to have some clean environment to start with. In the 'old' days you had some simple SQL to do this. Since SOA 11g, it is more complicated. As I wonder, why it is so complaticated, I do not know.
Marcin Izdebski created a very nice article about purging the SOA 11g dehydration store, including some straight foward script that does the trick! See: http://www.emarcel.com/soa-suit/152-deleteinstancessoasuite11gwls