I suggest using the tab-completion that is there to expand the entire host.
You can also have a look at the /oban command in the ChanOp Script at http://www.weechat.org/scripts/source/stable/chanop.py/ which seems to support custom/default banmasks.

/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:

jdhore!*@*
(Which bans the nick only)

My 2 suggestions are to either have it ban:

!@full-host-here

or

*!user@full-or-partial-host

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.