If this were to be enabled I’d suggest that it be a temporary, not a permanent block.
You’d have to renew it every month or two (or whatever time period) and there would be several warning messages prior to when it ends so that you have the option of renewing it (eg: “your block on arachnids is ending soon, do you want to continue blocking this taxa Y/N?”).
This would allow the block to be put in place, but would also remind people that they’re making an active choice to ignore a branch of life and might help them overcome their phobias by gently asking them if they’re ready to start seeing some of the subjects of those phobias.
I’d also suggest that if it’s enabled a bold marker is put on the user’s home page (that they see, not anyone else), saying something like, “You have temporarily blocked X taxa from view.”