Can't stop replication sp_dropdistpublisher responds "there are subscribers defined"
Have you tried using this to enumerate the list of subscribers to find if it thinks there are some defined? Notice that I'm just starting in sp_dropdistpublisher and reading the code to see where it checks before deleting. Just an idea.

--
-- Name:
-- sp_MSrepl_enumsubscriptions
--
-- Description:
--  Enumerate subscriptions on a database
--
-- Returns:
--  0 == Failed
--  1 == Succeed
--
-- Security:
--  public, db_owner check
--  Requires Certificate signature for catalog access
--
-- Notes:
--  Used by the UI to generate a list of subscriptions
--