Just killing the com.apple.dock.plist from the finder may not work because the system holds the current settings in memory and will just write them back. 


You could try and delete it from Terminal.app


----------
Update.

Go to  ~/Library/Application Support  folder in Finder and remove the folder:

`Dock`  (this holds a dock database )

Then in terminal type and run


`defaults delete com.apple.dock.plist;killall Dock`