Reviewing through your description it appears these users might have admin permissions set somewhere in your instance. For more information on permissions you might find the Managing Permissions section of the Jive Documentation helpful.
However, if you would like assistance investigating this behavior, I would recommend submitting a case in your customer group here in the Jive Community so that the next available support engineer can take a look directly into your instance.
thanks for replying.
I had indeed opened a case in my customer group but no one replied - that's why I also asked a question here.
A member of the Jive support team has now taken the ownership of the case though, and he'is helping with it.
Thanks again. Have a good week.