Can I prevent IP theft?

I am the owner of a business with a number of employees who have access to my servers through their own devices. I am concerned that there may be intellectual property theft. How do I ensure that my employees understand that all information, including sales-led business leads are owned by the business? I am worried that any employees who leave or are made redundant may use company information to start their own business or help them get a new job. Is there any way I can prevent this from happening?

Employees should have a dated employment contract which they read and sign. It should state that Intellectual property (IP), including copyright, inventions and designs created by the employee during their employment, is owned by the business.
The contract should also contain provisions protecting “confidential information” which might include marketing plans, sales or pricing data, customer or supplier databases, technical information, software, formulae, inventions and anything in any format marked “confidential”.
It should also provide that the business owns all records made by employees during their employment wherever and however stored, and it should prohibit unauthorised use or disclosure of confidential information.
On the termination of their employment contract, employees should be required to return IP, business and confidential information in their possession, and permanently delete it from all devices, keeping no copies. The employment contract should also require employees to notify you of their business contacts and opportunities and include reasonable non-solicit or deal clauses on termination.
Although ex-employees must not copy or retain your databases they may create their own from memory, or from directories as you cannot protect publicly available information.
Take practical measures – include fictitious names (“seeds”) in your database. If they receive communications, assume your database has been copied. Ensure your use of confidential information is consistent with its confidence. Mark it “confidential”, limit and police access, and password protect it where appropriate. Finally, notify employees of your policy permitting you to monitor e-mails. If the worst happens, take advice and act quickly.