Lemmy is open source, meaning that anyone can use it for whatever purpose they like. Realistically there is nothing we can do against that. The sooner we accept this reality, the better.
In my opinion, the main goal of Lemmy as a project is to challenge proprietary social media like Reddit. That wont be achievable if we try to exclude anyone from using Lemmy. Of course that doesnt mean that we have to agree with their opinions, and block tools exist exactly for this reason.
Also, I think if these extremists leave Reddit, that could eventually lead to their deradicalization. Reddit has no real incentive to deradicalize them, because more users and more activity means more ad money for them. But fediverse instances are usually run by volunteers in their free time, who have less incentive to permit such toxicity.