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You can only delete those files which you create yourself. If these are system files you are trying to trash, you'll need special permissions to delete them. Be careful though, if you trash the wrong files, you'll leave your system nonfunctional.
My software Force Empty Trash, or the competitor, Mac OS X Cocktail both can trash files created on the boot hard drive and override the permissions that may lock those files from deletion. Cocktail can trash more than my software can. But I would only use it for trashing stuff, and not cache cleanup, nor any other tool, without consulting a Mac expert. Cache cleanup of system cache can be very dangerous, and cause applications not to be able to launch anymore, including system applications. The link to my software*:
http://www.macmaps.com/software.html
I also have reference there to where you can get Cocktail.
- * Links to my pages may give me compensation.