Then you just set your authToken low

From a security aspect you can control the idle & token, but admins have no way of clearing browser remember blanks, so really this 4.5.x method is
more secure; as it avoids even remembering a username and the other functionality is settable.
I wish I could find that RFE, for enabling autofill on that blank for 4.5.x versions (as you want) ... but you may just want to see where this is at after v5 comes out as there's a ton of changes to the login screen anyway & autocomplete=off isn't appended anymore.
Do these as the zimbra user:
To change just the text of "remember me on this computer":
1. cd /opt/zimbra/tomcat/webapps/zimbra/WEB-INF/classes/msg
2. vi ZmMsg.properties and search for 'rememberMe' edit the line or comment this out with a #
3. vi ZhMsg.properties (for the html client) again search for 'rememberMe'
4. tomcat stop/start
Now the checkbox will still be there-so...
To remove check box all together you have 2 options.
1. cd /opt/zimbra/tomcat/webapps/zimbra/
js (jetty instead of tomcat in 5.0)
2. gzip -d -S .zgz Ajax_all.
js.zgz
3. Search for the following setting: _12e0.showRememberMeCheckbox=true;
4. Change to _12e0.showRememberMeCheckbox=false;
5. Save this File
6. gzip -S .zgz Ajax_all.
js
7. tomcat stop/start or a full zmcontrol stop/start
OR
1. cd /opt/zimbra/tomcat/webapps/zimbra/css
2. vi login.css
3. append this line to very top of the file #rememberMe { display: none; }
4.tomcat stop/start
Not prefect for a lab environment, but personally I'm happy with session idle timeout of 1 day (so people can't leave themselves logged in overnight/weekends.) And auth token I usually set to 3 days.
Bug 7958 - Disable "Remember me on this computer" on login screen
Of course everyone will forever inter-mix terms of auto-complete with blank auto-filling & what can be done server side vs browser password remembering etc; it's just one of those things you can't help.
Bug 9253 - Explanation needed for "Remember my Login"