@Mike: I am still not sure which version are you on 5.6.2 or above. 5.6.2 has a feature where one can hard delete a user. If you are on 5.6.2, what you did should have worked, unless you have modules that have put hard referential integrity constraints on Users table. One way to check which modules have Foreign Key Constraints on Users table to run this query, which I obtained from http://blog.sqlauthority.com/2007/09/16/sql-server-2005-list-all-the-constraint-of-database-find-primary-key-and-foreign-key-constraint-in-database/

select Referencing_Object_name, referencing_column_Name, Referenced_Object_name, Referenced_Column_Name from
(select Referenced_Column_Name = c.name, Referenced_Object_name = o.name, f.constid from sysforeignkeys f, sysobjects o, syscolumns c
where (f.rkeyid = o.id) and c.id = o.id and c.colid = f.rkey) r,
(select referencing_column_Name = c.name, Referencing_Object_name = o.name, f.constid from sysforeignkeys f, sysobjects o, syscolumns c
where (f.fkeyid = o.id) and c.id = o.id and c.colid = f.fkey) f
where r.Referenced_Column_Name = f.referencing_column_Name
and r.constid = f.constid
and Referenced_Column_Name = 'UserId' and Referenced_Object_Name = 'Users'
order by f.Referencing_Object_name

Note: This query does not make any data changes has no performance impact. Look for any suspicious tables, this should not return more than 10-11 tables.

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Ash Prasad
Director of Engineering
DNN Corp.

Two tables that jump out here are Blog_Blogs and Store_Addresses. If your users have entries in the any of those, you won't be able to hard delete users. 

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Ash Prasad
Director of Engineering
DNN Corp.