I guess you will need to join UserName on Users and aspnet_Users. Here is a script I got from Charles (not sure it was Charles) that deleted orphaned users. Maybe that helps:

DECLARE @UserName varchar (50)

--get a cursor to hold all the orphaned users
--that are in aspnet_Users table that are not in the DNN Users table
DECLARE users_cursor CURSOR FOR
SELECT au.UserName FROM aspnet_Users au
LEFT OUTER JOIN Users u on au.UserName = U.UserName
WHERE U.UserName is null

OPEN users_cursor

-- Perform the first fetch.
FETCH NEXT FROM users_cursor INTO @UserName

-- Check @@FETCH_STATUS to see if there are any more rows to fetch.
WHILE @@FETCH_STATUS = 0
BEGIN
--delete the user from all the aspnet_* tables that it may be in
--one at a time to avoid referrential integrity constraints

delete aspnet_Membership where UserId =
(select am.UserId from aspnet_Membership am inner join aspnet_Users au on am.UserId = au.UserId
where au.Username =@UserName)

delete aspnet_Profile where UserId =
(select ap.UserId from aspnet_Profile ap inner join aspnet_Users au on ap.UserId = au.UserId
where au.Username =@UserName)
delete aspnet_UsersInRoles where UserId
in (select uir.UserId from aspnet_UsersInRoles uir inner join aspnet_Users au on uir.UserId = au.UserId
where au.Username =@UserName)

delete from aspnet_Users where Username =@UserName

FETCH NEXT FROM users_cursor INTO @UserName
END

CLOSE users_cursor
DEALLOCATE users_cursor
GO