Bruno,
I have a similar issue but in this case, had to apply Chris Hammond's scripts to remove page-level skins by a certain provider. I don't want to mention names but a site I'm working on has their very good 5.x era skins applied to a 6.x site. Chris' work at http://www.chrishammond.com/blog/itemid/2614/how-to-clear-all-page-level-skins-in-dotnetnuke.aspx very much saved loads of time. With this I'm able to take back over setting host/site level skins.
However, one of the old skins can't be removed, nor the containers associated with it. No red X appears to uninstall these.
If I try to repair the install, an duplicate Index exception is thrown.
I have to mention the name of a very good company in this context to give more info and I won't do that. The fault isn't theirs, the fault is not having uninstalled an incompatible version of their product prior to the site having been updated before.
OK, to the question -- short of extending Chris' work and getting into other DNN skin settings in the database, is there an easy way to fully uninstall a skin extension after such an upgrade oversight?
I can give sensitive details in email, again, prefer not to mention product specific stuff in a public post.