Not that I would block them, since I think our users have the right to decide for themselves, but they are also highly radicalized.
The owner of that instance has on multiple occasions been ok with his users making calls to violence. See the link below of an example of one of their users making a call to violence which the owner of the instance defended and left up at the time (and this wasnt the first or last time)....very questionable instance sadly.
https://shitposter.club/objects/547c55f0-f50c-4105-8732-013e41c19a46
You can... or you can ask your user to move to a more inclusive instance? You can also follower their RSS feed or use subscribe if you just want to watch their account but not interact.