How to unregister message as spam/ham?

[Teemu Likonen]

I've been using Gnus' spam package for a while with bogofilter. So far I
haven't found a way to unregister messages as spam or ham. That is, undo
previously registered messages in bogofilter's database.

There is some indication that this is supported. For example, in
customize group "spam-bogofilter" there is variable
spam-bogofilter-spam-strong-switch with default value "-S". In
bogofilter that option unregisters message as spam.

How do I actually use this feature?