If you haven't got one, buy a cheap 1/4" hex drive security bit set - probably about a fiver from Screwfix or Leyland paints or other slack-in-quality tool vendor.
Choose a sacrificial six point star bit, that's a little bit too big to fit into the mess you've made.
Hammer it in to the mess hole and proceed to unscrew using the 1/4" inch driver.
I've successfully used this method a number of times.
If you're lucky, you'll come away without having trashed the security bit.
That's an interesting solution, I'll try that myself next time.
It's happened to me a couple of times I just used a hacksaw/grinder to cut two flat edges on the round head so you can get a purchase with a spanner/pliers/mole wrench etc.
It's a general engineering solution to a wide variety of problems.
The correct tool does not fit -> use the next-best thing that does, aided with a hammer.
For example, working on an old car with nuts and bolts that are badly rusted.
The correct socket will no longer fit on the hex.
Solution: Use the next-smallest size socket ( try the old Imperial ones too ), and belt it on with a hammer.
The rust will flake away, and the smaller socket will drive it's own grooves into the 'good' metal.
Of course, the outcomes can be variable.
Best outcome is the nut comes off.
Otherwise, it shears.
Or the sacrificial socket becomes a permeant part of the vehicle.
