It is pretty straightforward to purge Global Parameters programmatically using the Revit .NET API. The GlobalParameter class provides a few methods to help check whether it is used by dimensions, elements, or other Global Parameters. The three nice methods are:
GetLabeledDimensions()
GetAffectedElements()
GetAffectedGlobalParameters()
If they all return nothing or empty, we can safely think the Global Parameter is not used and can be purged out from the model. Or safer, we can check whether the Global Parameter has value or not. If it does not have real value either, it is really unused then.
Global Parameters came out late. They behave well. That is pretty nice and natural.