In law the website is considered like a home for the owner, at which they may discrimminate against the people they do not wish in. Donations unlike subscriptions don't confuse this issue.
In general I support the principle of evicting problem makers, however standards of what I constitute a problem are different to the website owners... as said its easy to mis-interpret people intentions or show them the failure of their ways.
I have some reasons why mean, disrespectful people can be a good thing, and that is to reveal them for what they are and to motivate others against them, but retaining them on a site after warnings only reveals a tolerance of it.