To what end?, if you're trying to keep the users from modifying the date and time, you can do it setting up restricted accounts on they're machines, the process varies depending on the OS, and the true level of security also varies. Still your best option is probably to read date and time from a server that the users have no access to. You can also synchronize the date and time with a server, there are some articles on how to do this here in the FAQ section, just do a search on Title containing ($) Time.

>how can i deactivate the time on the taskbar ?