What’s the best way to deal with intentionally mean and/or dishonest posters on the internet?

There is no easy way.

It kind of comes down to you.

I dont want to say be thicker skinned, because it depends on what the situation is.

If you are being abused, then google how you can defend against this.

Contact your ISP and ask how they can look after you. Do anything, but put up with shit from others.