disabling individual prefixes in input-method 'latin-9-prefix

[Ralf Fassel]

GNU Emacs 23.1.1 (i586-suse-linux-gnu, GTK+ Version 2.20.1) of 2010-07-05 on 
build17

I use
  (add-hook 'text-mode-hook '(lambda () (activate-input-method 
'latin-9-prefix)) )

Is there a way to turn off the prefix feature for some chars, like _ and /?

The Emacs manual suggests to type the accent twice (works for _, but not
for / (you have to use /-space for that one)), or to turn off the
input-method temporarily, but these methods are unhandy.

Since I hardly use _ and / prefixes, I'd like to turn them off completely.
Any hints?

Thanks
R'