It is possible it was possible to modify them in 9.10 with BIMCollab, but in 9.12 BCF Live Connector supports only standard BCF API connections and that is the limitation there. I believe BIMCollab also dropped support for the old-style connections recently?
https://github.com/buildingSMART/BCF-API can be used to discuss if the viewpoints should be made mutable in future BCF versions.
And one possibility of course is to contact BIMCollab and see if they could make it possible to delete viewpoints.