Sure they can. I don't see why someone famous shouldn't be allowed to not have to see people who are clearly mentally ill. If someone says something absurdly damaging to your reputation with zero evidence, just block them and move on.
The worst that happens is they say that "Oh no, this person blocked me, what a coward". Famous people are not beholden to the people to have to see everyone even if they don't want to. Celebrities have rights like anyone else and on social media the right to block anyone they want is absolutely extended to them.
Does it get tiring? Sure, I would imagine, but by replying, you're giving them exactly what they want. Attention. By blocking them, you deny them the attention they crave and they can no longer comment on your posts without making another account, which is a pain in the ass to do anyways.
As much as I hate Kenny, I think he's been handling this poorly tbh. Just block the morons and move on. Commenting on this shit only will only embolden them further.