Hello,
I think your question is not very specific, but the choice of term 'stealing' lead me to thinking that your employees are selling your projects/part of them to other people or using them in their own profit.
If you mean by "Stealing" re-using chunks of code in other projects or exporting them for later use is not considered stealing. In contrary, good programmers know how they save hours, otherwise wasted in implementing features they made before, by preparing well debugged methods, classes after finishing each project. That is why experienced and skilled developers tend to deliver features faster with less bugs/issues while saving company money/resources.
However, if someone is reselling products paid by your clients or using code that your team of developers worked hard on it on a personal profit. Let me put it this way, I don't think you want to keep that kind of person in your company.
TL;DR
It is a good practice to re-use OWN previously implemented code in different projects. That saves money and effort and it is a win-win situation. If someone is selling the team's effort to gain profit/without the company's permission. That is seriously bad.