@HistoPol
I know what #whitelisting is general, but how would I go about this on #Mastodon?

The whitelist, if I don't want to do everything manually (no-go,) would need to be "intelligent" and able to discern the platform s.o. is using for his handle...
I don't use Mastodon, so I don't know for sure. Some people have mentioned that there is a whitelist mode that is called "limited federation mode" or something like that. The admin would have to turn that on since it is for the whole instance.

If you don't want to use whitelist mode, people have been talking about blocklists that can be imported into Mastodon. I am not familiar with how they work. Maybe someone who uses Mastodon could answer this one?