I would not engage these people. I would delete and block.
It may seem funny to you or maybe you feel like you have
to stick up for yourself but the truth is that on the internet,
you can't win if they really wanted to do battle with you.
They can be totally anonymous, create a bunch of fake names,
bomb your vids with negatives, post replies directly to the
people who comment on your videos and harass them too.
So now your faithful subscribers don't want to post and be
harassed as well. It's up to you, it's YOUR channel. But I
do NOT recommend engaging undesirable people.