You can discover who is doing it with Lemvotes

But no, you can’t block them. Here in the real world (as opposed to the dystopian centralized platforms that have largely supplanted public discourse), it is not possible to isolate a specific individual and deny them access to information provided freely to the rest of the general public.

Should their public engagement rise to the level of harassment, there are legal options you can take to compel their restraint. But downvoting everything you do does not rise to such a level.