I have a block list, some folks are on it. It isn't a static list, it will grow but you've got to be really "special" to get on it.
Honestly, I have no problem blocking someone and don't think its a sign of immaturity at all. (Talking about people with whom you refuse to engage in conversation might another issue.) Around here, and on other forums, there are some who I need to avoid. In order to do so, I block them. But it isn't something I take lightly.
Frankly, for my own sanctification blocking someone might be the most mature thing I do if I sense there will be spiritual trouble down the road.