Scenario
1 . Lets say john gets banned and makes another account.
2 . Lets say he then gets ip banned for ban avoidance.
3 . He then changes his external ip and static ip.
4 . He then boots up his computer again and makes another account
Conclusion
John has avoided all his bans and is now ready to start doing banworth
stuff again.
If it becomes a big issue you can always ban the entire subnet, which no amount of modem resetting will change beyond. Of course then there are proxies...
Bans on any service work like this, most people don't care enough(or know how) to evade.
Heh, I'm a hassle on IRC, but on the forum I'm generally a good member. Never evaded a ban from the forum but I've evaded countless IRC bans.