/ban $nick is very easy to evade. In fact, it's as simple as parting the
channel, doing /nick some-other-nick and re-joining.
Currently it makes the ban:
(Which bans the nick only)
My 2 suggestions are to either have it ban:
The latter can't be evaded without the user disconnecting from the server,
changing the username in their IRC client's settings and
reconnecting/rejoining (or using a shell/proxy, of course). The former can't
be evaded in any circumstance in many cases, or in a few, can only be evaded
by the user using a shell, a proxy or resetting their DSL modem.