Login/Logout of Sharepoint Portal 2007

I am new to Sharepoint 2007.

I noticed when testing with different userids that if I login with userid 'A' then switch to userid 'B', close the browser, open a new browser session, the userid 'B' is still logged in. Is there anyway that a user can be logged out of sharepoint when they close their browser Ultimately If I opened a browser from my machine (userid 'A') I would want it to log me into Moss but not if I login from another machine.

Thanks