Often employees approach their Line Manager or perhaps a colleague to provide them with a testimonial/reference, on the basis that the testimonial will likely be more favourable. However, it is important to make clear that such a testimonial does not necessarily represent the views of the Company.
The easiest way to deal with this is to have a company policy which states that all requests for references or testimonials should be directed to a nominated department (HR?) or person (Director?). This is simple enough to do, or we could assist if you prefer.
Ensure that everyone is aware of this company policy, and it is probably a good idea to remind them by regular email - perhaps monthly.
Then some thought needs to be given to the content of the reference you are willing to provide. If you don't tell the truth, new employers could have a comeback. If you do tell the truth, past employees might not be entirely happy. The easiest solution is to keep references truthful but simple.
There is a widely held belief that you can't say anything bad about a former employee. Not true. If, for example, an employee was dismissed for theft, then you are perfectly able to say so. In fact, not disclosing something so important could come back and bite you in the proverbial.