kldunload -f does not work

[[LoN]Kamikaze]

I'm using the radeon driver and thus suspend kills the system. I'm
trying to get a 'lame' suspend by sending xdm a -HUP signal and killing
the drm and radeon modules with 'kldunload -f'. However kldunload
ignores the '-f' option on my system. The output is always the same
(with and without '-f"):

kldunload: can't unload file: Device Busy

The way I understand the manpage '-f' should force unloading anyway.