For example, in this case I don’t want HD runway textures I previously bought and historical airports like Kai Tak. I don’t seem to be able to disable or remove them. Any ideas?

I am not sure but it seems they have not enabled the functionality, maybe because the Marketplace was not launched to please certain third-party developers. Like we cannot install the scenery or aircraft it is all streamed but I understand we will in time be able to do this. I hope there is functionality; there are numerous addons from 2020 that I do not want and I have some conflicting scenery.