I have mountain lion and don't have the option to delete the cache either.
To get round this: At the desktop, hold option while you click the Go menu, and select the Library.
Within the Library, drag the Messages folder to the Trash.
While you're in the Library, you could reclaim some wasted space if you like.
Quit all open applications, then delete the Cache folder, and within the iTunes folder you could delete all those iPhone/iPad/iPod software updates.
Now restart.
🙂