You can not stop this. You could perhaps change your password with your provider and check your local machine for a virus. But otherwise your getting spam, most likely because you allowed a log on to a site using your Facebook or other social media account and they took your contacts as part of that signon.

You have to be vigilant with social media logons and check exactly what they are asking for access to.

I have never used social media on that machine . Have checked for viruses too and there are none. I do not open the links so hopefully it should be ok but I would be grateful for any other suggestions.