After forwarding the misguided notifications, the guys from Disqus were kind enough to send me a detailed response that makes sense.
As mentioned before, “kris” is my shortname. What’s happening is that these other sites are using my shortname. Indeed, “Kris” is a very common name and I was lucky enough to obtain it. The thing is, that in Tumblr you can enable comments by typing in a shortname rather than registering — this isn’t proper, but people do it anyways.
These users have no control over their comments as a result, only I do. Luckily Disqus provides a feature called Trusted Domains, available under general settings.
Trusted Domains allows you to specify which domains you want your Disqus forum to load on, effectively blocking the other sites who you’re receiving notifications for. In summary, this functionalilty restricts usage of Disqus to certain domains. By default, Disqus doesn’t restrict the usage to one URL. That’s why I entered my domain as a trusted domain.
Problem solved, thanks Disqus!