The FCP "delete generated library files" only deletes certain files. It does not delete analysis files. If you *carefully* inspect the .fcpbpundle in Finder via right-click>Show Package Contents, you can tell what is consuming space. If it's the analysis file folder, FCP, as currently designed, does not delete that -- whether on a local driver or a NAS.
If FCP does not delete render files after you do "delete generated library files" with all the checkboxes enabled, it really should do that. However in general there is no network-specific code in FCP for NAS cases. It issues the same filesystem I/O commands and the NAS is supposed to handle that. If your non-deleted files are render files, let us know.
If it is analysis files, that can only be deleted manually with Finder (which can entail some risk if you aren't proficient with such things) or with Arctic. I don't know what the Arctic support policy is for NAS-hosted FCP library files. You'll have to ask them.