Truworths is harassing me, calling me every hour

Truworths has been harassing me with calls looking to collect unpaid instalments, after I have repeatedly told them that they have the wrong person. After I blocked most of their calls on my cell, and I also told one of their people to stop calling me, they are now calling me on my work number, which I don't know how they even found it. This is very annoying and is bordering on stalking!