Get a less sucky version of python for mac:
http://docs.python.org/2/using/mac.htmlDownload pywallet by clicking on the blue word pywallet.py on the page
https://github.com/joric/pywalletOn the same page, click on the blue word README and read some stuff
Close Bitcoin, back up your wallet.dat
open a console window, change to the directory where you downloaded, type
pywallet.py --webGo to the super-secret web site at
http://127.0.0.1:8989/Under "Delete a key from your wallet:", put in the hash of the transaction you want to delete
Alternately, type "pywallet delete transaction" into Google and go to the first result.