Is there a way to disable the Sapphire.Restore.Defaults action. [snip] I guess this is possible using a SapphireActionHandlerFilter, but they are only available on property editors, and not on the Section part e.g.
You can use action handler filters on section. I have fixed the missing action settings for sections issue in the sdef editor.
Would it also remove the Clear action next to a property editor?
No, there is no means to remove content from the property editor assist popup. Rather than trying to suppress these actions, you can instead no-op when deleting the list entries that aren't supposed to be deleted within your custom list property binding.