There's the inbuilt nick-completion that you can use without a mouse, just type the first few letters of a nick, and keep pressing TAB until the right one appears.
Failing that, using this line in Options/Mouse under 'Nick List':
editbox -fa $$1 $+ : $editbox($active)
This will remove the ability of double-clicking a nick to bring up a query window, but will put the nick followed by a colon at the start of the editbox, even if you've already typed something.
