Network,IP and Junk mail

I'm suffering from below problems.

1. I have LAN, with around 25 workstation connected. But some of them are using the pc's for playing game instead of work. I want to control from my pc, as admint to able to see their activity, and
if i found they misuse the pc during work time, i can sutdown their pc. Is it possible? what software that i need to use?

2. We using outlook express, email for work. But many time we receiving junk email with content advertisement. it's disturbing us, since flooding to our mailbox, probably some of them are viruses.
is there any tools or software to block it.

3. We sometimes using yahoo or MSN for chat. but many time other user are disturbing us by keep sending message, and i surprise that he knows my IP address. I don't know how can he saw my IP
address, and how can he, some time sutdowning my pc from internet. Is there any software that i can use to see others IP address trough internet? and how it work. please let me know.

Anybody can help me, please?

 

About 1:
If you are the admin: strip all authorizations from all users (unless you're using win9x/me, alas security is not a feature). There can only be one admin on any system: You.
Remove all unauthorized software and install management software (you running windows? is there an ads tree or NT domain?), start by installing tools like vnc to be able to take over a session en support remotely.
Get a log analyzer.

2: do you have a central mail server (exchange, smtp)? If so install an antispam solution (like spamassassin) and an antivirus scanner on that server. Plenty of options here.

3: get a decent firewall with network address translation facility. Configure your lan using only private range ip addresses (10.*, 172,16.*, 192,168.*).

Get a book on network and internet security. It seems you're missing some of the basics here