Delete All for the Junk folder works in iOS 7 for iCloud, Yahoo, and Gmail accounts, last I checked. In iOS 6, it only worked for the iCloud account.
I suspect iOS 7 will work with most IMAP accounts, but have no way of testing this.
If you are on iOS 6, or this is not working for your account, your best bet is to get your provider's email app from the App Store (if available) or use the webmail site to delete your Spam/Junk mail on a regular basis.
If neither of those work, you'll either have to do it the hard way of selecting each email, or do it on a computer. Sorry.