Again, I have not seen this problem, but have the following suggestion. If we think that something is referencing the dictionary/schema, we should be able to check that in each of the potential locations. So filter on your schema in each of the following locations and see if WFA shows any items.
1) First ensure that you have removed the data source (Execution)
2) Delete the Data Source Types (Designer)
3) Check Reservations (Execution)
4) Check Workflow Design Elements (Workflows, Finders, Filters, Commands, Templates)
5) Check Data Source Design Elements (Cache Queries)
6) Check the Schema to make sure it is not "locked"
7) Try to delete individual dictionary entries from the schema
It looks like a lot of things to check, but it should be quick and easy to see if something is referencing the dictionary/schema.
Mike