I suggest that you get each employee to sign a copy of an agreement along these lines:-
"You agree and understand that as a condition of your employment you are responsible for taking all reasonable steps necessary to secure the company’s network, computers, information and communications systems against unauthorised access and/or abuse. Any deliberate violation of this condition will result in immediate disciplinary action.
You further agree and understand that you are responsible for complying with current UK and EU legislation where it affects the use of our computer systems and equipment. The company will provide you with the means to understand current legislation as if affects your work for the company - either in written form or in the form of training at your place of work, or an alternative location designated by us. Any attempt - successful or otherwise - to make illegal use of the company's computer systems may result in disciplinary action and/or prosecution.
The following actions may also result in immediate disciplinary action:
1. The deletion,copying, or modification of files and/or data created by other company computer users, or by any other company or individual with whom the company has business dealings.
2. The use of company computers for the conduct of personal commercial activities.
3. The use of company computers for personal entertainment purposes.
4. The use of company computers for immoral, illegal or unethical purposes of any kind.
5. The deliberate use of a company computer or other device in a way that damages or disrupts any computing system, alters it normal performance, or causes it to malfunction.
6. The unauthorised removal (for any length of time)from the company's premises of any item of company computing equipment or file storage media."
Please understand that this is not necessarily a definitive text. I am often asked to advise clients on these matters, and my suggested wording varies, depending on specific circumstances. You should satisfy yourself that your text deals with your own concerns - we can't accept any liability for your use of my suggestions, although you should find that I've covered most of the common issues.
Make sure that each employee signs a document to signify that he.she has read and understood the rules, and ensure that you provide adequate information and/or training about any relevant laws. You want to make sure that nobody can later claim 'I wasn't told what the law was, so I didn't realise that downloading 50 Gigabytes of music from a torrent was illegal'.
Good luck, let me know if you need any further help or advice.