In a console, do
- you should see output similar to
24801 ? Sl 0:00 xmms
24855 pts/1 S+ 0:00 grep xmms
replacing 24801 with whatever number you get.
What you're doing is identifying the process ID belonging to xmms, and then sending the system a direct command to kill it.
To prevent it happening again, right click on the xmms player, -->visualisation--> visualisation plugins, highlight the relevant plugin and uncheck the 'enable' box.