Did you try pressing on the brake pedal?
Seriously, "rmmod" is for removing kernel modules, not for killing processes. "kill" is for killing, stopping, or sending other signals to processes that are running. If a "kill -9 PID" (where PID is the Process ID number) as root won't kill something then there could be several reasons. Usually it's a "defunct" (also known as "zombie") process.
Sometimes programs/daemons will spawn a child process which when killed or dies sometimes will not get properly cleaned up by the parent process (poorly written apps sometimes are the cause). Trying to kill the zombie child process may prove fruitless, but if you kill/stop the parent process the zombie should also disappear.
Also I have seen times if you try to kill a process that is in the act of performing I/O on removeable media that may be damaged it can hang the process and appear not to die. In those cases I find that manually ejecting the media allows the program to exit. I am sure there are more that I can think of. Most of the time killing or restarting the parent will fix you up though.
Don't know if this is what you are after. It's really hard to tell with the very limited information you gave, like what process you were trying to kill and what the process was doing when you tried to kill it.
