Actually, I can delete most users from before the update, there are only some users (perhaps very, very old users) which cannot be deleted. So I think I can only not delet users which where added a couple of updates ago.

Does that sound strange?

I also found this script elsewhere on the forum.
Does anyone know if this has any sideeffects:

DECLARE @VUserID VARCHAR(512)

SELECT @VUserID = UserId FROM ASPNET_USERS WHERE UserName = 'promero'
DELETE FROM ASPNET_USERSINROLES WHERE UserId = @VUserID
DELETE FROM ASPNET_MEMBERSHIP WHERE UserId = @VUserID
DELETE FROM ASPNET_PROFILE WHERE UserId = @VUserID

DECLARE @IUserID INT

SELECT @IUserID = UserID FROM DNN4_USERS WHERE UserName = 'promero'
DELETE FROM DNN4_USERPORTALS WHERE UserId = @IUserID
DELETE FROM DNN4_USERROLES WHERE UserId = @IUserID
DELETE FROM DNN4_USERSONLINE WHERE UserId = @IUserID
DELETE FROM DNN4_PROFILE WHERE UserId = @IUserID