Cruising the Internet can expose the computer (yours) and the network (yours) to viruses, backdoor trojans, hijackers, spam, porn, and insidious worms that can do great damage to your productivity and your business. Warn your employees and then have your IT professional check the traffic. If you don't have an IT policy yet, draft one and implement it with fair warning. It is grounds for termination, if he understands the policy.
If he is pestering other employees by sending unsolicited email and attachments, that is another matter altogether. To allow this to continue could open your business (and you) to a lawsuit for sexual harrassment. (Even if the content is political, often there is a pornographic element to it.) He MUST be terminated, in that case, no warning necessary.
Just make sure that you document the infractions.
I've been there. I overlooked a salesman's cruising on the Internet because I do it too, but it took me 12 hours of work to rebuild a his computer after he was fired for not doing is job. And I still haven't gotten all the junk out of my network.
Backdoors and highjackers are all over the Internet and can wreak havoc with your company files -- even if there was no ham intended. When your computers slow to a crawl, youwill know what I mean.
We agree. This must never be allowed. You never ever use company e-mail for anything but business. That will get you fired quicker then anything else.
Using company e-mail to send a letter to the editor for example, is exactly the same as using company letterhead. It is saying that you speak for the company. You are not the company spokesperson, unless you are the owner of course.
Remember the first rule of business, thou shalt never embarrass thy boss who signth thy paycheck. The boss may even agree with you, but companies rarely want to take public political stands unless it is for sound business reasons.