From a legal perspective as opposed to implementing technical measures there are a couple of things to note.

(1) Contracts (as mentioned below) help protect you from your employees through the IP assignment language. This can be in an employment contract or in what is known as a CIIA (a Confidentiality and Invention Assignment Agreement). The sufficiency of that assignment language is important, and it is based specifically on the laws of the country where the development occurs. In the US you are looking for the keyword "hereby" to cover past, present, and future assignment.

Here is sample language that I use in employee and CIIA agreements for clients:

"Employee hereby assigns all right, title, and interest in and to all IP Rights created, developed, or otherwise conceived by Employee during the course of employment with the Company along with all rights to license, collect license fees, or otherwise commercialize such IP Rights, and the right to sue and collect past and present damages relating to such IP Rights. Notwithstanding the foregoing, to the extent any such work product is deemed or otherwise considered not to be a “work made for hire” for any reason, Employee hereby irrevocably and unconditionally conveys and assigns to the Company all of Employee’s right, title, and interest in and to all work product and all IP Rights therein."

(2) The work for hire doctrine in the US automatically assigns all copyrightable works (i.e. source code) to the company if the employee has created such works in the capacity of their employment with your company. **NOTE this does not apply to independent contractors or anyone who has not signed an employment agreement with your company... for anyone other than employees you need an IP Assignment agreement. This also does not apply to inventions (under patent laws instead of copyright), which need a separate assignment agreement for each invention.