You don't.
You let it fall off the catalog.
That's it.
You can either place an anchor or lock it, so that no one can post it it and/or it wont bump up anymore.
move it to another catalog?
Are we talking about the same thing?
We are talking about threads on qresearch, right?
>are you ever able to return them to the catalog?
That's impossible as far as I know, because certain information is lost, as well as all media content.
Here on /qresearch/ some media content is saved onto wayback machine because of this reason.
As soon as a thread falls off the catalog, it gets archived (if archiving is enabled).
You can put an anchor, that will stop anyone from bumping or, or - that's how it's done on here, lock it. In both cases bumps are not possible anymore.
That way it falls faster, but it takes quite a while, here on /qresearch/ I think it takes around 1.5 months.
You can of course also archive it onto wayback machine, and then delete it, but that way it won't make into into 8kun archive.