apt-get upgrade stopped, how can I remove packages before continuing upgrade?
 

apt-get upgrade stopped because of a lack of space. But now I can't remove any packages with apt-get because it stop suggesting me to use "apt-get -f install" to solve the problem.

But of course, since I have no space left, "apt-get -f install" doesn't work.
(I have a separate /usr partition, so I can't free space by freeing my /home... :( )

How can I remove packages then?
Is it safe to use "--force-yes"?

You should really have studied the release notes for Debian 4 BEFORE you started the process. I always take a printout and study in detail and have it beside me during the process - here ends the lecture :)

Ref; http://www.debian.org/releases/etch/releasenotes

Ideally you should be using aptitude for the upgrade. According to the afore mentioned release notes removing previously downloaded packages located in /var/cache/apt/archive will free up space. This can be done by doing apt-get clean or aptitude clean. I seriously suggest the you follow the first paragraph above.

I use Ubuntu. (but it has aptitude too of course)

I had already tried apt-get clean but it didn't solve the problem.
I didn't think about aptitude.

Anyway, I solved the problem in a quite simple way in the end:
I used synaptic, used the "broken" filter, deselected the broken package to remove it and "apply".
(However, deselecting the broken package forced me to remove a lot of other things. I don't seem to miss them yet, but who knows...)

Another useful tool that comes from the release notes that would pickup this situation is the command dpkg --audit

you also might want to install localepurge it will free up quite a bit of space in man pages and documentation that is for languages other than you speak/read..