Orphans

[gapan]

What about automatically blacklist slackpkg in slaptget.rc ?

That seems like a good idea but it won't really help. slackpkg is never going to be installed as a dependency of another package and running slapt-get -i slackpkg will always install it anyway.

[damNageHack]

How delete the orphan packages?

Maybe you could create a list of packages that are currently and actually installed. Then do a "proper" fresh reinstallation with only the packages still wanted? Sure you should consider about the "forced" dependencies, without them the system will not work any more (see above the other comments to that issue).
Take a look here into my howto done long time ago: viewtopic.php?f=30&t=430&hilit=reinstall
I guess it is not exactly that what you are looking for but it will be a good workaround and "you know what you are doing" with the *manual* reinstallation method.