Hi, there aren't any settings in Kentico to change this behavior when on-line marketing features are enabled.
Currently, the contact is assigned to one or more user accounts if these accounts don't exist in the system yet. So, if a contact arrives to your website and registers a NEW user account, the account will be assigned to it. If the contact logs out and creates another user account, the account will be assigned to it as well. This would go on and on and one contact would end up having more than one user account.
However, if a contact arrives to your website and logs into some user account that already exists in the system, the contact will be linked with this user account only. If such a contact logs out and logs into another already existing user account, the contact will be linked ONLY to that second account instead (from the time the contact logged into this second account).
To sum it up - if a contact logs to already existing user account, it will be linked to this account only (so one contact will always have one user account only). If a contact keeps creating new user accounts, he will be linked to all of them.