Quit all applications including iTunes
Now open iTunes and see if your iPod appears in the source bar. Try ejecting it now.
If that didn't work, try ejecting it from the finder (Mac) or PC. In the Mac - right click on the iPod and click eject. In Windows, go to my computer and right click on the iPod and click eject.
If that still hasn't worked, feel if the iPod hard drive is spinning. If it isn't, just pull the plug - no pun intended.
If the iPod's hard drive is spinning, try closing and reopening iTunes and see if your iPod ejects.
If that's still a no go, try resetting your iPod by holding down the Menu and Center buttons for 10 seconds until the Apple Logo appears.
If that's still a no go, shut down the computer. If that still didn't work, then just wait until your iPod's hard drive stops and then pull the plug.
I hope that helps you out.
Cheerio
Searay