In version 1.3.6p2 the method Net::HTTPStreamFactory::unregisterFactory() is declared but does not appear to be defined.
A simple work around of course is to use:
Poco::URIStreamOpener::defaultOpener().unregisterStreamFactory( "http" );
BTW, a personal pet peeve is the idiom used in URIStreamOpener to create the singleton in the defaultOpener() method.
static SingletonHolder sh;
I realize that this method makes no claim to be thread safe and indeed it is not. In a multi-threaded environment it can, and undoubtedly will, return an invalid object at some point.