zimbra editin : 509 opensource. os:redhat as 5

My users often close ie tab(not close entire ie , only close the page of zimbra in ie7 or firefox)but not click logout buttom to exit . So they found when they did this , the second time they only need to input the url and click enter , it login automaticly. This make them a little worried about the safety of their mails . I tried in administrator console , check/uncheck "cos--option--warnning when users leaving",but no use . some suggestion ?