Ok, I tried the hard way.
I erased all users from Joomla, except the few needed for administration purpose.
But then It appeared another problem, about wich I was not aware: all users from HikaShop, wich has orders attached were not erased! Ok again, I can understand this. I went to HikaShop and erased all orders, for erasing the users. Now, I have users only in HikaShop, I mean I haven't in Joomla too, and in HikaShop I can't erase them! I have no button for this, none!
So I erased users from Joomla for nothing, because I can't do it anymore now.
However, now I realize that volens nolens, I need to have orders in the memory of HikaShop, but I need also to work in guest mode with the posibility to use the same e-mail without login. I mean to be a real guest mode, how it used to be for some time!
It is very difficult to do this?