Maybe this is already known, but I found it by accident. There´s a filter to view particular developers but not a filter to hide them. But if you add a minus sign to their names and paste it in the search box (top right corner) you can hide them very easily. Multiple developers can be hidden at once just by adding a space between their names. The only constraint is that only one word from their complete name can be used with this method. The rest of the standard filters can be also used together with this manual filter.
For instance to hide both ORBX and Aerosoft you should write:
I just used some of the best quality delelopers in the example because in my case I need that filter as I purchased several of their addons via their own stores, so I need to hide them as Marketplace does not detect them as installed because they use their custom library folders, so the “Hide Owned” option is not working in that case. As I already own them I don´t want them to be displayed again in the list.
On the other hand if you are also tired of having a lot of low quality addons populating the lists while browsing the Marketplace you can use this method to hide some of those developers as well. Try not to use very few letters in the filters to avoid that airports using ICAO codes that contain those letters are hidden by mistake.