About your comment that you didn't know it was possible to own the entire .dev domain:
Not sure how much you know, but this ending is the top level domain (TLD) and it is possible to purchase them like the secondary level domains most people are used to. But it's hard to buy them, expensive, and you need to provide a bunch of services for it.
Google and some other companies do own a lot, for example there is a .google TLD. Countries have their own, and CERN has one.
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
About your comment that you didn't know it was possible to own the entire .dev domain:
Not sure how much you know, but this ending is the top level domain (TLD) and it is possible to purchase them like the secondary level domains most people are used to. But it's hard to buy them, expensive, and you need to provide a bunch of services for it.
Google and some other companies do own a lot, for example there is a .google TLD. Countries have their own, and CERN has one.