Earlier quoted context omitted.
Trademark law should handle this (at least in some countries). Register a trademark and then send a cease and desist to the fork as they are creating confusion in the marketplace by distributing something they created using your registered name. Perhaps consider using Apache 2.0 license, section six has language that specifically addresses trademarks.
A lot of open source devs are allergic to promoting their work so owning the trademark won't help them. For example, the above developer could have created their own official GUI and put it in the app store but they won't. Realistically the value is in GUIs, SaaS, marketing, etc. not in backend code.
I do a fair bit of promotion on YouTube[1] which has resulted in a vibrant community around the software, and I am now creating an official, free GUI[2].
In this specific case even if there had been a trademark in place, the person in question had not used the name of the project at all, but had decided to aggressively market it in community spaces.
Ultimately after much reflection and experiencing first hand how easy it is for people to abuse (in my opinion) the option of deliberately hard-forking and introducing breaking changes to funnel people towards a paid piece of software (in this case, a GUI) that will _only_ work with their hard-fork, I concluded that preventing this kind of ecosystem fragmentation is more important to me than an "OSI-approved" badge.