As I've learned from this question, [bracketed] processes listed by the ps aux command are kernel threads. So is there a way to kill them from the command line? If not, I guess the reason for that is to save the user from a higher risk of getting a kernel panic, right?
You can not kill kernel threads, or any process that is blocked in the D state, because signals are only delivered when the kernel returns to user mode. Aside from the technical limitation of signal delivery, killing a thread in the middle of kernel code would corrupt the system as the kernel code may be holding an important resource at the time, such as a spin lock or mutex, and killing it would prevent those resources from being released.
If you have a process that is stuck in the D state for a prolonged period of time, then you have a kernel bug. See https://wiki.ubuntu.com/KernelTeam/KernelTeamBugPolicies for tips on reporting it.
