The message
from Ian Stirling contains these words:

In some cases, just the right size of flat screwdriver can work.
In other cases, it breaks the pin off, and you can use a conventional
key.

And, for future referrence, if you want a simple way to screw up a Torx
or Hex socket to stop someone undoing it - tap a tight-fitting
ballbearing into the recess.

--
Skipweasel
Pay no attention to the man behind the curtain.