There isn't a way to specify that a task does not necessarily represent something actionable. For instance, an Epic is a collection of tasks that need to be worked on for the epic to be resolved. However, an idea is just an idea, it can be rejected or embraced and converted into an epic.
Create a generic idea tag for ideas that are in Phabricator.
Alternatively, we could say that ideas should not be on Phabricator, if this is the case, where should they be?