If you try to empty the Trash but get a message that says that you can't, try these steps.
1 Open the Trash folder by clicking its icon in the Dock.
2 If any file displays a tiny lock graphic on its icon, you'll need to unlock the file to trash it. Select the file, press Command-I, click the General disclosure triangle in the Info window, and deselect the Locked checkbox. Then empty the trash.
3 If none of the items in the Trash have a lock icon, most likely an application is preventing you from dumping the file. Quit the application that the file is associated with (for example, if it's a partially downloaded file, try quitting Safari), then try emptying the Trash again.
4 If that doesn't work, a system process could be holding a file hostage. Try restarting your computer, then empty the Trash.