Nothing ever done will prevent aliases. If somebody wants to intentionally fuck shit up via an alias, it's going to happen. No matter what preventive strategy you want to go with, it will interfere with people just trying to make a regular account. You can't limit the number of usernames from a certain IP because sometimes there are brothers/sisters that share an internet connection or a whole house of skiers living together that share a connection all with usernames. You can't set up a certain number of posts or thread viewings because sometimes people are new to the site and want to ask a question right off the bat and having them jump through those hoops could possibly deter them from using the site.
basically... aliases are just something everyone has to deal with.