RE: Open a LogIn form on another users session when I terminate his active one
Could you please elaborate what you're talking about?
It seems that you use somebody else's credentials to log into Windows and connect to the running session, therefore you see applications running in the session. I assume you close AX client and start it again and you're surprised that it still uses credentials of the current user. But what else do you expect? You log into Windows as UserA and therefore you're logged as UserA in AX as well, because AX uses Windows authentication. If you want to use AX as UserB, log into Windows as UserB and not UserA. It's technically possible to use RunAs to start AX client as a different user, but while it's useful for testing, it doesn't make much sense in normal usage.
Then you mentioned some form, but it's not clear what you want it to do. Maybe you didn't realize that AX uses Windows authentication.