In the Release Notes for Mendix 2.5.5:
Ticket 8319: Fixed hidden tab being selectable with key navigation.
I guess you need to upgrade to a newer version.
You cannot avoid this, since the information is present anyway, just not visible. If you check the css code below (which is taken from Firebug), you can see how a tabpane is made invisible: by including a ' style="display: none;" '. Removing this information in Firebug immediately makes the tabpane visible. This is simply the way Mendix works.
div class="dijitTab" lang="" dojoattachevent="onclick:onClick" dojoattachpoint="titleNode" role="presentation" dir="" widgetid="mxuiwidgetMxTabContainer0tablistmxuiwidgetMxTabContent6" style="display: none;"
end CSS code
The only way to make sure a user can not see this information is to prevent it being sent to the client. You could configure entity access, or make a new form for this user role which does not include the tabpane you want to hide.
In 2.5.6 you can set the form with tabs on not editable, but if you then create a dataview in a tab and make a column editable you can inline edit the record.
If I recollect right this was a bug and resolved in later editions. So the only sollution would be to upgrade to a newer version. I hope that somebody from Mendix still can recollect this bug and answer this.