But seems like I can't. I currently have /, swap, /usr and /home partitions, so I thought I'd resize /home a bit to expand /usr

But while I can umount /home no problem, I'm unable to unmount /usr since it always says it's busy. I've logged out all users except root, killed X and several other processes, but it seems that something is always using it.

Since basically all processes seem to use /usr, is there any way to umount and resize it, or do I have to reinstall it (this time with just / and swap and maybe /home partitions)

Hi marenkiapina,
there is certainly no need to reinstall.
You can for example boot a system from CD and do all the manipulation you want.
The distro I linked to does also provide a real nice graphical partition tool (qtparted) to help you with the task.
Be very aware that you can permanently damage your system if you make a mistake there. The program will warn you before making critical changes.

Good luck,
- drowstar

Use the 1st install CD and boot up rescue mode.