Buy a Blank Firing Gun, I have a Desert Eagle in 8mm blank, which is loud as a bastard. I also have an air rifle (most powerful it can be without a licence - 12ft/lbs) and a Beretta 92FS (nickel) which is also rather powerful
I suggest just pulling a blank firing gun, as they would shit themselves when you pointed it at them, and as they're running away, fire a few rounds at them, hopefully they won't fuck with you again
btw, i wouldn't use any of my guns to hurt someone (let alone take it out of the house/property) but if you're lucky, you might pull it off good luck
...Josh