Can't delete .plist files from Trash

When you have problems to empty the trash, rm -rf ~/.Trash/* in the terminal does the job for you.

Edit :

You have to keep in mind that there might be multiple .Trash directories.
For example on a USB stick or a external Drive, OSX will create another .Trash at the root of the drive. So you might need to use rm -rf /Volumes/DriveName/.Trash.*

If none of the other solutions work, you may have to turn off System Integrity Protection in Mac OS X. This was a solution found suggested on Apple Stack Exchange as a method for editing the Keyboard-en.plist files for the Press-and-Hold Character Picker in OS X. Their solution referenced MacWorld UK here. To do so:

• Reboot your Mac into Recovery Mode (Restart, hold ⌘-R until the Apple logo appears)
• When the OS X Utilities window appears, go to Utilities > Terminal
• Type csrutil disable and hit Enter
• Reboot your Mac
• Empty your trash. The .plist files should disappear
• Reboot into Recovery Mode and this time enter csrutil enable into Terminal and then reboot again to resume System Integrity Protection on your Mac.