Certainly, you can do that, but why be on a forum if your going to ignore people?
The same reason why the forum bans users. Sometimes people show up that spam the forum with ads or scams. Banning them keeps the forum from getting bogged down in unwanted spam. Similarly, since I don't want to wade through pages of salt pictures and idiot theories about Martian pizza parlours, I block out users do nothing but post them. These people don't ever post anything of value, so nothing is lost.